Hi Bee124 ☺ welcome to Beyond Blue.
I think it's really good you're reaching out and that you've done some research too.
I'm Bipolar and would suggest to wait till the inbetween times before approaching any sensitive areas as the stress and pumps high in mania and lows are tragic depending if he's type 1 or 2, so it could end in arguments.
Maybe you could approach it in a way of asking him if he thinks anythings bothering him and depending on his reply you could maybe say you've noticed at various times a shift in energy and mood and go from there. You seem very caring. Assurance that you're by his side would go a long way too which is clear that you are.
Another choice could be to talk to your GP that may have some ideas too or possibly ring Mental Health also might be of some help.
I hope it goes smoothly for you and be happy to hear how you get on ☺