So I mentioned to my mum that my boyfriends kitchen bothers me. The inside of his microwave is covered in old food and so is his fridge. I noticed his floor was dirty in the same spot for at least two weeks but he took care of that. there's sometimes trash on the floor circling the bin randomly, I'm not sure if they are recyclables or not. It just seems odd to me. Everywhere else is fairly tidy. Anyway my mum says I should mention it, however I feel like I'm crossing the line since I'm not living with him.
It depends on how much time you spend over there. If you're over there a lot then he should keep it a bit tidier, in my opinion. But if you EVER even broach the subject of moving in together then you definitely want to bring it up.
If it's a serious relationship then you aren't crossing the line. You could either bring it up to him, and explain that you would like him to be a bit more cleanly. Or, when you're over you could clean up the messes yourself. But you definotely aren't crossing the line. Leaving things sit for too long (especially food in the fridge or on the floor) can be a health risk so you're really looking out for both of you!
If you want, you can offer to help him clean his kitchen a bit. If he says "no thanks" or otherwise doesn't accept your offer to help, it's best to just drop the subject. There's plenty of time to nag each other after the wedding.
You can casually bring it up. When you're in there, say somehing like 'this is quite messy, haha' see how he reacts. If he says something like 'yeah, haven't had time to clean haha' or 'you think so?' You could offer to clean with or for him.