I would say do the surgery, esp if your colt is starting to act out.
I have gone through this same thing with my colt.....this was our only foal that we raised on the farm and he was quite the stinker from the get go. I did wait to see if his "berries" would drop and I had the vets out I think 3 different times before I ended up having the crypt surgery done. Frankly Rascal got to the point where he was trying to rear & strike (and mind you, I don't tolerate bad behavior, so this was not just a horse being bratty)......he as well as all of my horses know their boundaries, but hormones take over and he became a bear.
Anyway, looking back, I wish I had done the surgery even a year earlier......I had it done late last summer and he was 2....but again, after getting advice from people, including the vets, I thought, well we can wait and see if his other testicle drops. It never did and the surgery ended up over $800........but you know what, he is a totally different horse now and I was very careful with his recovery and he is so much nicer now........ he had to be hand walked daily because he couldn't be just "turned out" to exercise and that was fine, after plunking down twice what I was quoted, I wasn't taking any risks with that incision opening up. Healing went fine, no problems, not issues......... and he is much happier as am I now, knowing I can go out to his pasture and not get bit, kicked, or reared up on. I know after reading this people are going to think my colt was just not disciplined, but you know what, he was and he still is, but he is much, much nicer now, like a totally different horse.
Hopefully this helps and I am sure if you do the surgery you will not be sorry, esp if your colt is starting to change his behavior...... good luck on what you decide and please know I have been through this, so if you want to contact me, I am here for you.