Budget? Sounds like a pretty big 12 year old. As stated above, he's going to grow fast. Is he strong for his age? If so, get him a good 12G, one that's not going to beat his shoulder to pieces. My first was a Mossberg 500 and I didn't like shooting it because it kicked the crap out of my shoulder. No reason not to go semi if he's going to use it for bird hunting, unless your budget is allowing something top shelf. Then, I'd agree with the break action.