R.J.
Well....
Probably the first is just to realize that SOME people are loners. And they're okay with that. Extroverts get energy from being with other people, and being alone makes they tired/depressed/cranky. Introverts get energy from being alone, and being around others too much makes them tire/ depressed/ cranky.
I'm definitely an introvert, but my son and husband are extroverts.
Don't get me wrong, I ENJOY people, but on my own timeframe. I have a few good friends that I trust completely and implicitly. Most of them are out of state, which suits me fine, because then I get to call them. And when I need alone time I get to hang up. ALSO I get to "visit" for a week or two a couple times a year, which suits me down to the floor. I'm utterly happy with my deep friendships.
Acquaintances are interesting, but I don't want to spend a lot of time with them or too frequently. And I interact with people all the time, and I'm one of those who can talk to ANYONE ANYWHERE ANYTIME about ANYTHING. I'm an introvert, but I'm not shy. I just highly enjoy my space and my privacy.
If your husband works, he's around people all day. If he's an introvert like me, then there is probably nothing more enjoyable than coming home 9x out of 10.
It's the 10th time that you CAN help. Try giving him a "day". Like a Wed evening, or saturday from noon - ____ (like 7pm or midnight). On that day he can do ANYTHING he wants. Meet up for drinks after work, go ________, whatever. Make plans without asking/checking with you first/ arranging babysitting. ((Introverts often need dedicated time where they can PLAN to interact with people so they can psych themselves up for how draining -even though fun- it will be. Also, when there are extra "hoops" to jump through -like checking with a spouse and finding out that day won't work, trying to find another time with said friend that will work with THEIR schedule, etc.- it just becomes too much "work". Because visiting will be tiring anyway -but still fun- adding more stress just makes it a no go)).
The OTHER thing you can do is to encourage him in his _________. Maybe he used to play basket ball, or swim, recite bad poetry before he got married and had kids. Maybe there's something he WANTS to do/learn that he just hasn't gotten around to yet. I often play the "lottery" game. Meaning: If I won the lottery, what would I be doing? It's helps cut through the miasma of daily life and find hopes & dreams. With a little dedicated time, and encouragement from a spouse, a person can still DO those things. Just slower (because they still have to work). Friends are a natural byproduct from doing things you enjoy, because you find other people who are ALSO doing what you enjoy.
I have to highlight something I've brought up a couple times. For introverts, being around other people is exhausting... BUT... it's still fun when you actually like the person, or are doing an activity that you like with them (like playing pool, or mountain climbing, or watching foodball with heart-attack food). Still FUN, just tiring. The two are not mutually exclusive. It just creates a challenge to work around. Pretty much for every block of time I spend socializing, I need about 3x that block in order to "feel like myself" / feel alert/happy/rested. So when you're blocking time for him, don't be surprised if a LOT of that time gets spent in solitary activity. That solitary time is absolutely VITAL to also being social. It looks very odd to extroverts though. Like we're doing the opposite of what we should. When we're not :) It's just that we have a different set of needs.
BTW... BIG HUGS and kudos to you, for wanting your husband to be happy, and for wanting to help facilitate that. More than anything else, I think real love is defined by another person's happiness being vital to your own. Not in a codependant kind of way, but in a "wanting the best" kind of way.