Hello Debbie, and a warm welcome t the forums.
Sometimes it may take a while for someone who has been in a marriage or relationship to decide that now they want to be with someone else, so I'm very happy for you, but he may not have wanted to disclose this to you, in fear of not accepting his condition, but if you love him and vice-versa then this is an avenue to get to know him more.
He may be slightly frightened to mention this to you, solely because others may have given him a difficult time, that's why he may be apprehensive, so you could just mention that you've noticed the medication and understand how he could be feeling, but you're there to help him and would like to know what he's suffering from and if there is anything you could do to assist him.
It's just about approaching this with the knowledge it could be something he's wanted to shield from as many people as possible and maybe scared of any reaction, but if you love him as you say you do, then you can ask him, one point is it's not good for someone who may have Depression of any kind to be asked question after question, that's what I always didn't like, cause I didn't have any answers, but when he wants to talk about it then let him have the floor.
You could possibly ask him if he's still having treatment, that's a fair question for you to know, because this might open up another door that he's closed to everyone else.
We'd really be interested to hear back from you whenever you're availale.