If he works all week, he may be feeling that the weekends are his only "down time" as well as his only time to see the kids for more than a very short time at bedtime, etc. That may be the root of his hating to be out and about on the weekends; it's possible that he sees doing outside activities with the kids as cutting into his time to be at home, to be with them, to recuperate from the week at work.
However, and it's a BIG however, that does not mean he should whine, complain and be a drag when he DOES agree to do family outings. Maybe you both need to sit down without the kids around, at a time when you and he are both calm and happy with each other, and talk about this: Does he realize at all that you see his behavior on outings as being a drag and a complainer? He might not; he might sincerely believe think he's doing OK just to be there and he might think he's being supportive when to you, he isn't. If you can calmly discuss this, and not make him feel as if you're attacking him (which will possibly make him defensive and he might start in with "At least I agree to go! What do you want?!" etc.), then maybe you can compromise.
Look at yourself too: Are you wanting to be go, go, go both days of the weekend? Will one day of the weekend do? Are you into big, busy, noisy and active family festivals and events, when he might be more OK going with the kids to a park or mini-golfing just as a family instead? Can you compromise, say, on your taking the kids to the big festival alone or with friends, while both of you take them to the park (or he does, alone) the next day?
Are you the one picking and/or arranging most of these outings? Maybe he should have more input, or even be the one taking the lead. Show him the newspaper listings for family-friendly events and places and let him choose. Or is he complaining that the outings are OK but take too much time? You find one that's shorter next time. Is he upset that there were outings two days in a row? Tell him you know that, and next weekend you can hold it to one day -- and would he be willing to be positive on that one day? Etc.
What's the root of his issue? Does he just not get into doing kids' activities and so he's bored? Some adults just cannot ever embrace participating in family activities; those parents are the ones who usually are more active with their kids once the kids are older and the kids share some of the same interests and activities the parent likes already. It's not a great situation for the other parent, or for the kids, frankly, but it's just the way some adults are; they are not used to being childlike and never do get used to it, but that doesn't mean they are bad parents. However -- yeah, another however -- he should make the effort at least sometimes, so that the kids SEE dad enjoying THEIR enjoyment in things. That's a point you may want to make with him: Children need to see their parents enjoying the children's enjoyment; it makes the children feel their parents think their (children's) feelings and choices are interesting and important. Even if the parent is frankly bored at that moment.
Also, you mention almost in passing that "Prior to having kids my husband wanted to stay at home too." Would it be correct that prior to having kids, you were the more outgoing, active, adventurous half of your marriage? If so, you knew what he was like as a person; having children has not changed that. You say you "don't know if I can change him on this." You probably can't change his fundamental self but you can possibly talk with him (and maybe have some of his dad friends who are more active with their kids talk with him) about why he should compromise and maybe do at least half the outings with you and suck up any boredom and enjoy watching the kids having fun even if he would rather be at home. And while you're asking him to compromise and lighten up and choose some activities with the kids that suit him better, be sure that you too offer to compromise and do more at home together at times too.