Jenarry, I think you should confront him. You looked at his text, ok that was naughty (i have done it too, though!) but the issue here is, what is going on with him and that girl. Maybe nothing, but you have a right to ask and a right to know. Tell me, if you secretly followed your boyfriend and you saw him with another girl, would you confront him? Of course you would, so you check his text, you find one that you feel uncomfortable with, do you confront him - hell yes! Don't do it accusingly, just tell him what you did, explain you should not have, but the issue is - what is going on, and the next issue is, why don't you two go out more together get a babysitter, if you can't, why should you miss out on the fun, surely you can both stay home and have some friends round? Be brave and ask him, otherwise it will fester inside you, trust me, I know....
Be strong and have a lovely Christmas x