What chance does he have to improve enough to play D1 in a year?
Does he have a very projectable body?
I think that needs to be considered. Juco probably makes most sense for projetable late bloomers (i.e tall and thin) or very raw but athletic guys who need more baseball experience or some mechanical overhaul.
So honestly asses how much room for growth there is. Sure he will work hard but don't expect magical improvement and ask yourself why he will make the jump this year that he hasn't done before. There is always room to get better but you have to be honest with yourself.
That means if you are 5"11, 190 and you already trained extremely hard and you are mechanically polished D2 or D3 might be the better option.
But if you have some realistic room for growth go for the juco option and work your butt off to get better.