Need some advice..

[TamTam]

Wasn't he happy on that trip you guys went on?

It's probably not good that he has nothing else to occupy his time.   It's actually really, really good for thought processes if you get away from it for a while and let the subconscious deal with it by itself for some time, before coming back to it consciously.  It provides new angles, creativity, and relieves mind fatigue.  So not only would breaks from thinking about it help him emotionally, it could help him make sense of it all and come to a firmer and more satisfying conclusion, too.

You might already know that, I don't know, but if he needs a reason to let himself get away for a while, you could tell him that.  If he doesn't need a reason, he just simply can't stop thinking about it, I'd say it'd be really good for him to have some kind of change of scenery.  Again, to relieve fatigue.  A fatigued mind just runs in circles like a crashing computer.  Can he go camping in his backyard?  Does he have a bike?  Skateboard?  Video games?  Books?  Does he have anything he can do besides sitting at home?  There must be something.  Even if it only gets his mind off it for a split second, that's better than nothing.

[Seshatneferw]

He comes to me for help and advice but I can only reassure him that I love him; I don't know what to say to him as far as advice goes.

You are already saying the most important thing, that you love him regardless of whether it's he, she, or something else.

Even when I tell him there's plenty of time to think about it, he says it's the only thing ever on his mind. I haven't seen him happy in weeks \=

That sounds all too familiar. A couple of months after I came out, first  to myself and the next day to my wife, there was a period of some three or four months when it was the only thing on my mind. After that it started to get better: for the next half-year or so I could occasionally get some work done, and over this autumn I've mostly found myself again to the extent that even my wife recognises me as me, not the incarnation of gender confusion. And surprisingly the world feels brighter in many ways than when this all started.

In short, things will get better, but it's going to take some time. Perhaps weeks, probably months, hopefully not years. But the odds are very good that the overall trend is up.

Oh, and about feeling like a liar, as you mentioned earlier. The way I was able to deal with some similar fears was to tell myself that it's ultimately about how I feel about myself, not how others see me. I made a deliberate effort to not care about whether others see me as male or female, and concentrated on getting comfortable with myself. This meant a critical approach towards a lot of gender stereotypes, including transgender ones, and it was hard. In the end it worked for me, though -- at least I'm pretty sure...

  Nfr

[Vexing]

The unfortunate reality is that if he's living at home with an unsupportive mother, he's always going to feel hurt and restricted - because it's her home and she is calling the shots.
I'm assuming that is the accommodation situation, that he's still at home with mom?
My suggestion would be for him to find a counselor and to take his mother along for several sessions, so that she can begin to understand why he feels the way he does.

And as much as it is wonderful that you are trying to help him out; this permanent fixation on thinking about being trans is something that only he can shake.