It really depends on the person and their situation. I know with little kids, parents often inject after because it's hard to get them to finish their meal. The general "rule" is to take your shot 5minutes before eating, so your insulin is working at the same time the food is making your bloodsugar rise.
90% of the time, I inject first. 10% of the time, I inject after. If I'm sick or for whatever reason not sure I'll be able to finish my meal because it's so large(rarely a problem for me haha), I'll wait to inject until after.
The problem I have with injecting afterwards, is sometimes I forget and that causes a HUGE problem later. I once had a lunch date with a friend and his gf, I decided to inject after the meal since I was headed home to grab my camera stuff for their couples portrait session anyways. Well, I forgot. Two hours later, in the middle of the shoot I realized I didn't take my shot. I had also left my kit at home, thinking I wouldn't need it (yeah the ONE time I do that..ugh). So there I was, a 30minute(if the traffic was light) drive away from home and my insulin...running high and getting a major headache. I had texted my bf when we took a breather and he started flipping out (I couldn't hear my phone so I didn't get his texts until I got home) wondering where we had gone, if I needed him to drive out and get me, etc etc. We shot for another 30minutes and then headed home. By the time I got back, I was 30!
Now that's the first time it's happened to me with meal insulin (i've forgotten my lantus a few times in the last year or so ugh after 16years) but it IS a risk of doing your shots after.
I would try the before meal shot but also talk to your endo about switching to after meals when your son doesn't feel he can eat it all.