Maybe he is genuinely too tired, especially if he has been working more/had a lot on his plate with uni. When he goes to bed it's probably because he feels ready to go to sleep and just wants to wind down, so maybe try to initiate sex earlier in the evening or at different times of the day when he is still full of energy.
However the best solution is for you to talk about it with him, not in the bedroom, not when you are trying to initiate sex, but in a non threatening environment such as when you are just chilling on the sofa, going for a walk etc. Don't confront him or say anything in an accusatory manor, just say something along the lines of 'I've noticed we've been having sex less often recently, and that you've been feeling tired a lot, can you tell me if there's something wrong?'
Then see where you can go from there and if it's something you can help/support him with and you are willing to be patient then I'm sure you can get things back to the way the were, or if it just seems as if you are no longer sexually compatible then consider ending the relationship. Just remember though, if he's someone you see yourself being with long term, a lot of couples will go through a dry spell at some point down the line, is it worth throwing away a relationship just because you aren't having much sex for the space of a few months? I know being with someone you are sexually compatible with is very important in having a happy relationship, but it probably is just a phase of low sex drive that he's going through and things will be back to the usual between you soon enough.