sorry Bezi doesnt sound good, could be just flirting though, but staying out all night doesnt bode well, my instinct would be to kick the living hell out of him, but this doesnt work, I know I tried, I had this same problem, so my advice would be to gather evidence, get it together, because he will have a thousand reasons as to why he has messages, and where he's been, blah blah, oh and the usual shouting you down because you dont trust him, checking his phone, so get your facts straight.
You can buy a little gizmo that will retrieve deleted messages, on these spy sites, but not cheap I dont think, take a mental note of what hes saying and doing, or write a little diary, will all fall into place either way. like tallies idea of changing her number to yours, but be careful it doesnt back fire, and try to stay strong this kind of situation can really test you, if he is hes a prick, and shes pathetic.
My brother gave me a peice of male wisdom when my little drama was unfolding he said that, "they will admit to something small, to cover up the something big! The rest of the time they will deny deny deny"
take care x