It sounds as if he might be so overly focused on his retirement that he is missing what's under his nose right now -- his own kids, growing up. The time will go very quickly and by the time he's ready to retire, they will be long past needing -- or wanting-- him to be at school functions or take them places. He may be buying his retirement at the cost of not really knowing his own kids during the years they most need him around.
You do need to keep doing whatever volunteering or committees you want to do, within reason. He needs to understand that doing those things, especially at schools or preschools, means you are much more plugged in to the school as a whole. In fact, I would urge you to be sure you DO volunteer at your children's schools as they move on into K and elementary.
I would get him to go into more detail here: Is the issue with feeling "stressed" by the need to be home early etc. because he gets flak from someone at work who doesn't like his leaving early? Does he have a boss who rides him hard about any requests to leave etc.? Or is the pressure and stress only from within himself -- is it a matter of his feeling that he MUST be at the office every minute to get certain tasks done? Does his job allow him to do any work at home, so he can put in an hour or two while at home with the kids when you're somewhere? Does he manage other people, and if so, could the problem really be that he is not delegating enough work to them, and feels he must take on everything himself "or it won't be done right"? You and he should explore the nature of his job, where and when he can do it, the other people involved and whether they are the reason for the stress, and whether he has perfectionist tendencies that may make him feel he is responsible for everything, all the time, at work.
He also needs to hear exactly what you said here: "Part of me feels like I NEED to do something because just doing laundry and running kids around makes me lonely - when I join in I feel better." You should not have to risk feeling down, even becoming depressed, because you feel he wants you to drop the things you are doing.
Evaluate how many hours a week you need him home at certain times, and whether it's enough for you and/or too much for him. Really look at where his stress over this originates. Do not deny yourself the opportunity to get out. Impress on him that as the kids enter school you will want to be more, not less, involved. And yes, get a helper, and keep that helper into the school year if you can.
My husband is a project manager and has five people whom he manages but he is very quick to come home early or take time off to do things with our daughter or allow me to do things (usually...things with our daughter!). But his position and the kind of work they do means he can often do a few hours of work at home here and there. And there is no one over him who would get on his case about moving his schedule around --his company, and the client for whom he works, are pretty family-friendly and he's been there a long time and is valued. But a lot may depend on the type of work your husband does -- and on his own feelings about it.