I stopped using the baby stuff for DD's clothes when she was about 2 months old and she didn't have any problems.

I'm still using it for DS though because when I washed his clothes in regular powder when he was about a month old he did come out in a rash. I'll probably give it another go when the bottle I have now runs out since he's nearly 5 months now.

All you can do is try - if he comes out in a rash (DS's wasn't anything nasty, it just looked like a bit of prikly heat on his trunk) just go back to the baby stuff.