Knock knees and toe out can be corrected to a degree in a young horse - 1 1/2 is getting a little late but it's not TOO late... yet. Barefoot trimmers are great for some horses and not for others, and the majority of barefooters don't know what they're doing when it comes to corrective work like your boy needs to get him straightened out. SOME do, I know two very good barefoot trimmers who do corrective work regularly, but most do not.
He isn't BADLY knock kneed, if he is at all. Toe out can cause the appearance of knock knees, as can incorrect hoof balance [incorrect hoof balance can cause both] but the problem with these leg faults is that they cause the hoof to grow and wear in such a way that it will not self-correct. He really needs corrective trimming if you want to straighten him out.