He's definitely not full Belgian, but very well could be half Belgian or something to that effect because he does have the build of one.
If they're both same riding ability, I'd say go for it. Just make sure you go to look at him and try him out first to ensure that it is the truth (Although I'm sure you already plan on that :) )
He looks cute, he looks like he has some more growing to do, but he is cute. Going to be a very stocky boy, so being a trail horse might just be best for him as he might be limited as to what he can do, if he stays as stocky as he is right now.