Take it to a vet, or have someone experienced in probing snakes, sex both for sure before you try to breed them. Several reasons for this...
1- If you end up with two males you will have combat.
2- A lot of wasted time if they are same sexed
3- You may not know if they are the correct sexes even after breeding attempts... if they do not produce it may just be a failed attempt or they could be same sex
4- You will not know what to monitor or who to monitor without knowing sexes
If he is indeed male, then that's a fine size to breed at. If your snake ends up being female, then (assuming it is a Colombian) it <i>may</i> be too small/young to breed. It really doesn't matter with males just as long as they are willing and able