What type of special education placement and services should an Admission, Review, and Dismissal (ARD) consider for a student who was in an automobile accident resulting in a closed head injury with many changes in physical, cognitive, and language skills? What type of interventions might be addressed, and how can the school and the family help assist this student in his transition in going back to school?

Without knowing the specific cognitive, physical, and language needs of the student, it is difficult to know which interventions might apply. Also be aware that the services considered "standard" in one school district or state may not even be available in other districts or states. Special education services vary greatly across the United States.
That said, hopefully the initial testing given by the school district will shed some light on the specific needs to be addressed. Almost always, districts will test in each of these areas and will also consider medical evidence provided by parents and medical professionals in their determination of services.
Here are some areas to consider, but again note that all services may not be available within any particular school or district:
Physical Therapy: In addition to any therapies covered by insurances, the student may qualify for physical therapy services provided in the school. These therapies would be geared in helping the child navigate his or her school successfully (climbing stairs, walking over curbs, standing in line, participating in physical education, etc).
Occupational Therapy: These therapies would focus more on the fine motor tasks associated with working in schools. Can the student hold his pencil effectively? Can he cut using scissors? Can he cut in a straight line? Can he pick up small objects as needed in his biology labs? The occupational therapist works with teachers to determine the needs of the student in the classroom and then works to strengthen those skills through therapies.
Speech Therapy: Noting the changes in language skills, the speech therapist works to make speech more intelligible and relevant. Speech therapists work to improve all skills, from forming the sounds of letters, to forming words that can be easily understood by non-family members, to creating sentences that make logical sense.
Note: Due to a lack of resources nationwide, these three therapists are usually in high demand and have incredibly packed therapy schedules. Parents may find that there isn't enough availability in the therapists' schedules to provide the level of care they need and may feel led to seek additional therapies outside their school districts.
There are also things that can be done inside the classroom:
Additional time: From providing additional time on assignments to providing additional time on tests, students can be allowed more time to show their content knowledge.
Additional materials: Students may need to be allowed a dictionary, a calculator, a speech-to-text app, or any other number of aides to help them be successful. Parents and teachers should look together at the student's goals to determine which aides may prove beneficial in mastering content.
Modified grading: If a student's primary writing goal, for example, is to respond appropriately to a prompt, it may not be appropriate to penalize this student's spelling or grammar. These types of modifications can be implemented into his IEP and re-examined as the student progresses.
Adjusting the length of assignments: If maintaining focus is a struggle or if working for extended periods of time is counterproductive to learning, students can be allowed modifications in the length of their required work. Perhaps students complete 50% of their math assignments (or 80%—all depending on the child's needs). In this case, parents and teachers need to work together to determine the goals of each assignment and the abilities of the child in order to ensure the child is progressing in all needed areas.
An instructional aide: In some cases, a student qualifies for an instructional aide. This person remains with the student for the duration of the school day, assisting with all needed tasks. This is especially helpful for students who have limited physical abilities and need help with restroom or feeding skills, for example. It is also helpful for students whose fine motor skills may prevent them from taking notes in class; this person then records all notes for the student. Or in math, the student dictates the math work to the adult, who transcribes the student's work.
An alternate placement: This one really varies greatly around the nation. In some areas, there are schools for children with special needs that exist to serve only those students. In other areas, students with special needs attend one multi-age, multi-ability classroom led by a special education teacher, all housed in the same school as the general population of students. In this case, it is typically a goal that students with special needs will learn, work, and play with typical students as appropriate (lunch, art, even classes like science—depending on the student).
There are more services that exist, but overall the goal is to help provide the needed supports to help this child reach his highest potential possible. The most successful programs are possible because of excellent communication between parents and the child's teachers. With ongoing, informal evaluations and the ability to have an open dialogue about progress and struggles, the plan for success can be modified as needed, as the child's needs are always being reshaped.