Maybe there are no signs. I don't know. But I think the best way is to try to make him trust you enough to understand you won't make him feel worse if he tells you his secret. Put yourself in his shoes, he may not be proud of what he did to just say 'yes' if he has done it or say 'no' now that his mum no longer understand him enough to know what he can do.
In my view, if he has done it, he isn't proud about it and if he hasn't... Then he's probably mad at you for losing trust in him, making him reluctant about talking about it. Make him understand you don't wanna suspect him wrongly and that you just want to know simply out of care. He's too young to do such and it could be part of the reason they first broke up.
