Why don’t you wait until January to close the other UGMA account, and then simply deposit the funds in the 529 so you can get the 2016 tax credit? That way you don’t have to be sitting on a check for an extended period of time, with the possible complications.
The 529 account is a custodial 529, right? Who is getting the tax credit, you or your son? I hope you understand that this is your son’s money, and your son should be the account owner of the 529 (as well as the beneficiary, obviously).
There’s no hard and fast rule about how long you can hold onto the UGMA funds in the form of a check before doing something, but you need to remember that you are acting in a fiduciary capacity for your son. This means that you must manage his money in a way that is in his best interests. Leaving the funds in the form of a paper check, that can be lost, stolen, destroyed, etc., and where the funds are not insured or earning interest for an extended period of time, doesn’t sound like the smartest thing to do.