You say that you know something about construction...well, then why can't you two work together in the business? Even if you don't do the actual physical labor portion, you could act in an administrative capacity, scheduling his appointments, etc. and then you would certainly know if there is really any need for a 2 a.m. "service call."
Also, working together even in a limited capacity may bring you two closer and you are striving for a joint goal and income at that point. He'll also feel you're contributing to the household if that is a possible source of his current resentment towards you.
If he is resistant to the idea of you joining him or working as a H/W team, I would find out why. Then you need to sit down and find out what is really going on.
The change in his attitude towards you, the disrespectful actions, late nights out, are all red flags to me. I first found out my H (fiance at the time) was messing around when he told me he went to get his computer fixed at midnight on a Saturday night. Yeah, right. I had called him that night on his cell at the prearranged time he told me, he was cold and acted weird and was rude to me (though at the time I didn't know why). I told him at the time he was a terrible liar because I just knew in my gut the whole computer story was fishy. I then called one of my friends and said, "I know he's with another woman" just by the way he was behaving and his bizarre cold manner towards me. Turns out I was right. He tells me now that when I outright told him he was a terrible liar, even though I didn't actually KNOW or could prove anything at that time, guilt kicked in and his heart pounded like no tomorrow.
TRUST YOUR INSTINCTS. They are rarely if ever wrong in such matters. If your H has changed so drastically in a year, it's possible he's met someone who is offering him the admiration/adoration and something else he may not be getting at home. That 2 a.m. service call may be more of a booty call.
So find out what is REALLY going on, and if he's not willing to include you in his life and/or business, and work on your marriage together with a counselor, then I suggest you at least find a counselor for yourself and start a contingency plan to take care of yourself in the event things end.
Best of luck.
~Silverwraith