aw, i'm sorry. sounds like this is hurting your heart.
my suggestion is that you DON'T try to make him talk. maybe he just likes talking with you more when you're there, and feels put on the spot to *converse* when you FaceTime with him. he's only 7, after all.
and his relationship with his dad is just different. i know it's hard, but try not to compare and contrast. you don't really WANT a relationship with your son like the one he has with his dad, anyway, right?
rather than cast about for things to talk about, or even the tried-and-true 'leading questions' gambit, how about keeping your eyes open during your travel days for fun things to just show him. 'dude! i was walking through the airport and had to pick up this shrunken head key chain!'
the head clearly needs a name and a back story. your son might just get interested in helping you find them.
or a postcard featuring a llama overlooking the ocean. what's the llama doing there? did his ladylove swim away? is he going to jump over a cliff?
or play him a bit of a song you heard that reminded you of him.
or make goofy faces at him.
or each of you get a bowl of ice cream and eat it companionably together without talking.
or just enjoy some pleasant quiet.
i think a lot of times that what feels like sharing to us feels to our kids like being grilled.
take the pressure off both of you and find non-verbal ways to FT.
khairete
S.