I'll bump this up for you, give you my take and hopefully others will chime in with their .02.
1) Volunteering is very important both clinical and community service work. There's already a great thread about shadowing. Get as much experience as he can. Most colleges have community service projects happening all the time. If he has trouble finding them, try Alpha Phi Omega which is a national service organization.
2) There is no preferred major. You have to take a set of classes (Physics, bio, calc, chem, organic, maybe others). So, many students major in biology or biochem. That's a good choice. IMO, however, schools will be even more impressed if you major in English and do just as well in your Sci GPA and Sci DAT as a biology major. Well roundedness is the name of the game. So, I say major in what you are interested in and what you think you will do well in.
3) Any leadership you can show is a plus. Anything unique is a plus. If he likes rock wall climbing? or literature or math then join such an organization. Show some leadership in the organization. Also, research can be important. Usually easier to get a research job at major universities however. There are summer programs and internships. Research can be a nice plus on the app.
4)Ideally you'd take the DAT in the spring of your Junior year right before you apply that summer. But you can take it whenever you feel prepared. Summer isn't too bad, but you should finish your application as soon as possible.
5) Community college. Tough question. I know several ppl that took classes or maybe 1 yr at CC and are now in dental school but I'm not sure about 2 yrs. I think it's certainly possible. The problem may be that when he applies he'll have 2 yrs of CC but only 1 at a university. This may not be that big of a problem. My opinion is (and I think the opinion of the application committee is) that while CC classes may indeed be as challenging (I hesitate to say that), I think it is generally believed that it is easier to earn higher grades due to the lack of competition in CC. This may or may not be true, but I believe this is the perception held by most ppl including the dental school which is what counts. So, my advice would be if going to a Univ is out of the question for now, do very well and earn very high marks in CC. And then do well on the DAT. On top of that explain why in your personal statement you chose to go to CC. If money was an issue, that's a pretty good reason. Thinking about it now, I guess CC puts you at a disadvantage compared to students from a Univ. You have to do better and maybe work harder than the other student. Otherwise, if both students show the same grades and scores, and only 1 would get admitted (perhaps an unlikely situation), my money would be on the university student as unfair as that may be.