wait until he get around 8-9'' then move him to bigger tank, it is safer that way. but I think t is best to move him to a 20 gallon then move him to the 40 when he grow to 5-6'' then move hm to bigger tank when he is around 8-9'', it is to keep him from breaking his back due to too large of a tank when young.
How does a too large of a tank cause a broken back?

