I would hold off on giving him meds right away. Your son is still very young and his brain is still developing, meds should be the last thing to considered after you have tried other non-invasive routes like behavior modification, changing his diet and supplementation. Honestly, any doctor who recommends meds first, from the get-go, for a child who is so young, is questionable to me. If it were me, I would probably try to find another doctor.
I know that there are a lot of naysayers out there but, in my experience with my son who does have high-functioning autism with ADHD-like behaviors, when we revised his diet and gave him certain supplements like magnesium and Omega 3, it seemed to calm my son's system down so he was able to focus and stay on task a bit better. There are a number of books out on the market about this subject. The ones I like are:
Healing the New Childhood Epidemics by Dr. Kenneth Bock
The Autism & ADHD Diet by Barry Silberberg
Something that you can start doing today is get all of the processed food out of your son's diet, if you haven't already. Eat like your grandparents or great-parents used to eat before food came in these nice little boxes and cans. All of those artificial colors in food are known to exacerbate hyperactivity. Also, limit his sugar intake. From my experience, with my son, sugar effects him differently than his little sister. She may be on a sugar high for an hour after eating some ice cream; my son will be on a sugar high for hours to the point where he may still be awake 11:30 at night (bedtime is at 8:30).
As for behavior modification, you can keep it simple by targeting 1 behavior that you would like to see your son do, like staying seated during circle time. Prepare a chart that he can put a sticker on for each day that he does what he is supposed to do. If he earns 21 stickers (21 days), he earns this special grand prize that he has chosen. To keep him motivated, I would also give him some mini grand prizes along the way (like going to the park of Friday after school or watching his favorite video at the end of the day) if he stays seated during cirlcle time. When you introduce the chart to him, really talk to him about what is expected of him and keep on reminding him several times a day about the rules of your new game. Also, if the teacher is willing, give her a copy of the chart as well so that she can remind your son of what he needs to do and what he will earn with his new behavior.
I just want to make one last comment. It always disturbs me when a doctor recommends meds right away for a child so young. To me, at least, it indicates that the doctor is very old school and indifferent to what your child's needs really and the fact that your child is still very young. Meds can be a good thing but there are always side effects that come with them. IMO, they should be seen as a last resort.
Wishing you and your son all the best.