M.C.
Yes, nurse him first on the side with the low supply. I would do a 2-3:1 ratio... Nurse him on the low side first 2-3 times, then switch to the other first. (You don't want to switch completely, or you risk overcompensating and becoming lopsided the other way. Lol.)
I also like to recommend pumping and storing the milk after each nursing session. It will only be a little bit, but it will help you build a good supply for the freezer. When I went back to work, my supply dropped BAD. If I hadn't built up several week's worth of milk, I would have been forced to supplement with formula while I tried to rebuild my supply. (It did level back out after just over a month...)
ETA: I wanted to add that you are *supposed* to nurse from both breasts for each session... But my DD rarely would. She would fill up on on one side and be done. I didn't want to switch her early, because I wanted to be sure she was getting the good hind milk. (You produce different types of milk... The first that comes out is fore milk, which is thinner and meant to quench baby's hunger. Then comes the hind milk, which is thicker and has more fats.) it sounds like you got lucky with your overall supply, like I did... So I just wanted to throw out there that it is OK if your son doesn't get both breasts every time he nurses. Just make sure to pump the other one dry too to keep the girls even. (Which will keep your supply awesome. The only issue I ever had with supply was that time when I went back to work.) :) This was per my DD's pediatrician and my lactation consultant... As long as your son is gaining weight, it's not a big deal.