So I just went through grad school in a triad (both my girlfriend and I at the same time, while our boyfriend was self employed) and holy shit it was hard. What I think you are running into is a combo of NRE for his new interest and how hard grad school is. The level of work and stress in grad school doesn't leave a shit ton of time and emotional energy and when something is new and fun and full of sex it doesn't take much energy. A long term relationship is a lot different. It's a hard thing about poly. Sometimes I had space for new people or long distance partners before I had any energy for my primaries. And thats kinda sucked. But when I was so close to losing my shit all the time I needed really simple, really easy things. It sounds like sex with you might not be super simple and easy while being close and spending time together is. One thing that makes sex really complicated really fast is guilt. I don't have any answers for you other then you guys should talk about this... which will probably also stress him out. Sometimes when my triad has a hard issue to talk about we try to make a really safe place (or we work with our therapist which, if you can find a good one, is a fucking gift to any poly family) where we all feel comfortable and have had time doing something that makes us feel close and connected and then try to listen very hard to what each other have to say... it works sometimes!
Anyway stuff like this really emotionally complicated it sounds like you have a clear head on your shoulders and a lot of love for your partners and you are working hard for them and your relationships. Great job!
Oh also Grinder is a hook up app for gay men. http://grindr.com/