i'd lean toward folding rather than raising with A6o preflop here (both are better than calling imo). i might open A6s but A6o is typically going to put you in tough spots when you don't flop 3 of a kind or two pair.
as played i probably raise to $2.25 on the flop to get value from weaker one pair aces. turn raise as played i think is fine. and river i probably make it something he cant fold to like $4.5. after he has lead three streets your river raise is actually pretty thin i think. the only worse hands he calls with are AT, AJ, A9, maybe some A7. there aren't many realistic two-pair combos that aren't aces up KQ maybe if hes bad enough). had he bet something like $3-4 on the river i would advocate smooth calling. but his shitty little bet means we can raise him 4-5x and he still gets good pot odds on his call if he has AT or whatever.