Heck.. my 4 month old colt thinks he's all man! He tries to mount my mares already!!
I can't answer your question because I don't have experience in your situation, but I can offer a few tips? First, he shouldn't be in with fillies unless his owner wants them pregnant. He needs to be taken out of there ASAP before it's too late.. but it very well could be already. Just because the owner hasn't seen him get to them and get the job done, doesn't mean it hasn't been. From my knowledge, stallions/colts capable of breeding are still able to produce viable sperm for up to 2 months after being gelded. Just some things to think about Good luck with him! Is this the buckskin/dun medicine hat colt?