I must admit him taking his phone everywhere sounds like he doesnt want you to see something
you must feel like rubbish right now :o(
this is prob gonna p*ss people off on here but if you are that concerned, you need to do something sneaky to find out,
follow him maybe, see where he goes, maybe trick your best friend into giving you info or what i reckon will work is................
is there any possible way you could get his phone?
this could seriously back fire but..... one night, maybe when he is in bed, text her from his phone... say something like
" i wanna tell ( your name) about us, i feel its unfair sneaking behind her back all the time, what do you think, text me back"
and see what she texts back,
now if she is confused and has no idea what you are on about then i guess there is nothing going on but....
she may reply with something else???