I'm inclined to think it is actually a chinese algae eater, not an oto, due to the the pattern on his back. His dots look pretty regular, while an oto's back in more mottled. If that's the case, he will definitely outgrow the ten gallon. They can also get vicious as they get older and may suck the slime coat off other fish. The fact that he is doing so well may be another indication - otos are usually delicate.
Time will tell you for sure - if he grows past two inches, he's not an oto.
If he is an oto, I'd advise a 20 gallon tank with at least 4 other otos. Keep a good supply of natural algae, not just algae wafers. If he's a CAE, I'd consider rehoming him now.