Weird, I feel the same way! haha, but I know how hard it can be having a husband not pitch in anymore when you're used to his help and even just his company. It can get both stressful and depressing for you.
I think a big part of why He might feel like this (coming from how I feel) could be like you said the heat and the fact that the kids are all out of school for the summer......I notice I stay busier, work harder and keep my kids on schedule during the school year but as soon as summer hits, we're back to sleeping in, lose routine and get off schedule. I hate that I do that but it's summer, you want to relax! And you mentioned he's not in the heat working but I stay home all day wiht the kids but just the thought of getting out in this ridiculous heat makes me feel crappy! I feel like a bad mom because I'm home with the kids all day but can't seem to find the energy to want to go run errands with them or take them outside because it's just sooooo HOT!
So if he's not like this year round, then I'd say it's the summer time and heat that makes him feel this way. I was just telling my husband how much I wish we could afford a vacation house up north to travel to during the summers to get out of this Texas heat, it's just so miserable here during the summers. Good luck to you!!