so a couple of things. Only a professional can diagnose because everyone goes through depressive eppisodes sometimes...what you are looking for is a chronic thing. the symptoms can also be kind of vague, and please dont take offense but it doesnt seem like he is really talking to you honestly about the symptoms because itis only things you have noticed not things he has told you. you can try asking him...since what he displays (some of them) can be symptoms, yes. the major thing is, if you think that he needs to talk to a professional then confront him about it or talk to someone around you (if you are in college or around social workers or counclers you can ask them about it and get their advice). just know that if he doesnt want to talk about it and get help (if in the end that is what he needs) then there is nothing you can do about it....people have to want help for themselves. as far as staying with him...if he has depression then he is going to have it for life...its a mental illness and while he might get treated and be able to manage it, this is something he would always struggle with. its not a "bad timing" kind of thing, and if you love him and want to be with him then know that this is what you are getting into and if you choose to remain in the relationship then you cannot walk away everytime there is an issue out of fear. You could get hurt, in relationships people get hurt, but you want to ask yourself to things: 1st are you in any danger (do you feel) do not stay in the relationship if it isnt healthy, because that is not okay. and 2nd if you are safe and it is healthy and he it turns out that he has some stuff to work through, stuff that he could have for his life (if its MDD) then are you willing to stick around for that. okay, lastly, how to act around him when you feel he could be depressed...be with him, try to get him to do something (even if it is a 5min walk). generally the first thing to try to get someone out of a depression is to get them active again...even if at first they are uninterested. try to get him to talk about whats going on, but in the end....your not a therapist, there isnt much you can do...do cant fix this (if there is a problem) for him...he would have to do it himself.
best of luck