Ok, just wanted to know so I wasn't telling you stuff you might already know. Was he wild or domestic bred?
I have a first gen domestic bred Stang that I have had since he was 1 1/2 and I am breaking him at this moment. I've been through all those bad baby habits with him. Mustangs are so freakin smart and can really take advantage of you quick, especially the babies because you end up not wanting to be too tough with them. But you have to be alpha mare, so once this little guy starts to get friendlier in the next day or so and you get to know him better, if he tries to bite again, you have to do something about it. Whether it's playful or not, that can be a dangerous habit to have. I'de carry a small crop with you (probably in the near future) and just smack him on the shoulder with it and say NO! If he tries to bite again. Let that register in his brain for a few minutes, then try and rub again. But now, I'm not sure about the trust thing....you dont' want to lose his trust fast, but he needs to respect you....if that makes sense sorry lol. Do you have a round pen or a small turn out? If you did, then that would be great because you could work with him that way to gain his trust faster. But with the babies and being "wild" there is no true fast way.