If he is really interested and starts mounting mares them have the vet do a blood test.
I have a gelding, and I use the term lightly, I got him as a four year old who had been gelded two months before I got him, he was gelded late because he had a testicle that did not descend, and they supposedly did the surgery to remove the nu-descended testicle. Well about five months after I got him he mounted my mare who was in heat, I did not think much about it. Well he kept acting more like a stallion, event got a cresty neck, started getting aggressive towards my other horses, and me. I had the vet do a blood test to check his testosterone levels and he was off the charts even for a breeding stud. Vet said he most likely had testicular tissue left from when he was gelded. I took him in and had surgery done to remove the tissue, after half an hour of digging around elbow deep in my horse the vet found some tissue, removed it. Next blood test done three months later his testosterone was still elevated, vet tells me he can go in on the other side and look for tissue, or even another testicle. I told him no I was not going through it again. So I have a cryptorchid which as long as I have him and am aware of his stallion tendencies and can deal with him is fine, but heaven forbid something happens to me there are ethical issues when dealing with this type of horse.