My son was a pretty successful first baseman and I agree with what has been written.
However, as a young person don't let him become completely focused on 1st. My son was only a first baseman which limited his opportunities. Could he play the outfield (he was left handed); yes, but everyone started viewing him as a first baseman.
There is many a coach that views 1st base as the postion that "anyone can play". As such; you will see many a player whose "normal" position may be elsewhere being put at 1st. If your guy has limited their skills only to first this can be a problem as they move forward.
As someone that has been down this road; if they are left handed, make sure that they become good outfielders as well as learning to play 1st. There will be far more options for good outfielder that can play first than a 1st baseman that is not viewed as an outfielder.