Folk, please talk to him! (Although I know it's hard if you can't talk on the phone).
It seems to me that it's possibly a case of miscommunication. Had he actually agreed to go to your niece's picnic and then he backed out at the last minute? Or was it more a situation where he knew the picnic was happening but hadn't committed to going and probably didn't think it mattered if he went or not?
Plus you said he e-mailed you to see if something was wrong - but you didn't tell him what was really wrong, just that you had a headache. It feels like he was trying to open the way for a discussion, but if you're not telling him what's really wrong, I don't know what he can do. (I'm not meaning to sound harsh - I've really struggled with telling people what's wrong in the past).
Also, it seems a massive jump from not hearing from him in the evenings as often as you'd like to assuming he's sexting other women.
I know you're worried what you'll hear if you talk to him - but if he's looking forward to going away with you soon, it seems unlikely he'll say anything that bad. Plus it's not as if you're feeling totally happy now and are worried about that happiness being shattered. You're sounding very unhappy, so wouldn't it be better to hear what he says and then at least you'll know what's going on?