he can move up to 3rd grade math right now or he can stay in the 1st grade classroom and do ALEKS during math time.
I would move him up to the 3rd grade math - that way he'll have his work officially overseen by a teacher and there won't be any question re is he capable of the work, completing it, etc plus there won't be any question of keeping him on track etc.
OTOH, if you are going to be watching over his progress in Aleks and he wants to work at home too and/or you think he wants to move faster than the pace of the third grade class, then I'd consider the Aleks option. My ds' one complaint about working 2 grade levels ahead in math (at school) is that even though he's moved ahead, that doesn't change the pace of instruction, and there are many times he'd like to move faster and doesn't want to repeat concepts that some of the non-accelerated kids in the class need to have repeated.
polarbear