I am right now a senior in high school and I am trying to get into the Air Force Academy. I went to the Summer Seminar last summer and they told me some information that might help you. One of my roommates was homeschooled and he asked numerous questions to our cadre (An upper classman cadet) leader.
Extra curricular activities: This compiles 20% of everything the admissions panel looks at. Very important that one is involved as much as he can be. This is a little bit more difficult for homeschooled students because high school clubs aren't an easy option. Some options are to be very active in your church. If you can help with children's ministries, outreaches, anything on a regular basis it will help. Call local charity orginizations and volunteer regurally. Some places you might check are your local food pantry, Local Easter Seals, town commons, Lions, KIWANIS, and local mayors office. Another option which would look extremely good is to start your own clubs that help the community. They are looking for participation and leadership. See what kind of leadership opportunities are available in places such as a mayors youth council, or a youth Lion's or KIWANIS program.
Sports: One thing I noticed that my homeschooled roommate was not in shape. Wasn't obese by no means but if he was to go through basic he would have a rough time. Being in a school system offers one the opportunity to play sports. Being homeschooled the option isn't so accessible. One has to find leagues to play in for any sport. You might try to find leagues by contacting local gyms like the YMCA for such information. Also call you city/town commons for other information. Sports teams are a great way to build teamwork, discipline, and communication. Also to stay in shape lift and do aerobic excersises on a regular basis.
Academics: Since the homeschool curriculum is possess a huge scope in subjects, difficulty, level, and grade system it is harder for a homeschooler to assess his academic perfomance and have it comparible to a school system. It is therefore imparitive that a homeschooler do very well on the SAT or ACT. One would also be wise to look into available college coarses to take which always looks good on a resume.
Final note: I was once homeschooled which shocks a lot of my friends because of the cliche of all homeschoolers being "sheltered" and lacking social skills. In my case and a vast amount of others the cliche couldn't be further from the truth. However, the cliche is there for a reason. Meet everyone possible and be involved with as many groups as possible. You will have to be interviewed by an Air Force Academy Nomination panel to receive the congressional nomination to enter the academy. One has to have charisma and good social skills. Build on them every opportunity you can. Try to join local plays to get comfortable being in front of people. Being homeschooled is no excuse for being "sheltered". Get out there an anyway you can. This is from what the cadre at the Air Force Academy told us and what my liason officer has told me. If you have anymore specific questions feel free to post and I will answer them as soon as I can. I hope I have helped and I hope your son doesn't have any troubles applying. Good luck.
Kinger8