I think it is a question of try it and see tbh. I don't think anybody knows how it will work out. Most of our dds friends are schooled and she plays after school sometimes or goes along to a youth club sometimes at weekends. You will find that your child is welcome to join after school activities with schooled peers, but I'm not sure about the ones organised by school, on school premises. If you wanted your child to have more opportunities to mix, look what is available in your local. We only have a small H.ed group here that only meet once monthly and quite often dd is too busy with other commitments to attend.
Personally, I would consider the quality of friendships rather than the quantity. A couple of close HE friends is more important than lots of groups, especially for shy or introverted children. In my experience, a lot of families start going to groups to meet people and often drop out to a greater or lesser extent when they have a created a friendship group. The difficulty is that if there are not many families, it may be harder for your son to find someone to click with.
Of the other families I know who don't do much HE group socialising, some have a large social network outside of HE (I am thinking particularly of Christian families who have a large church community). Others have a more geographically wide spread of friends, often met online, and do a lot of camps, either arranged privately together or "proper" home ed camps like Hesfes. This leads to short intense bursts of socialising interspersed with quieter times at home.
Is your son enjoying school, and has he made friends there? For an introvert, school can be tiring and stressful. This may outweigh any other potential advantages.
I am not convinced school is such a great place to learn social skills. For one thing, school doesn't resemble the social environment in the rest of the world very closely. For another, it's a case of "the blind leading the blind" if you are expecting a young child to pick up social skills from other young children whose skills are no better than his! Plenty of parents complain of the behaviors their children pick up from others at school. Your son might pick up social skills at least as well by going about his daily life interacting with you and other adults and occasionally children. For example, I trust that you yourself are able to eat meals without interrupting people, attempting to belch the alphabet, making disgusting remarks about the contents of your sandwich, or wiping your face on your sleeve, unlike many five year olds... so why would your lad not learn good social skills at home?
Some introverted children don't need or want to spend a huge amount of time with other kids. Whatever you can find may be enough, whether that is playing with neighbours in the street after school or seeing an HE family once a week.
You could try HE and see how it goes. School will always be there for him to return if he is unhappy with home education. But he might not miss it at all!
It's not as easy if there are few others around, but you can make the social thing work. Almost all of our friends use school at the moment as it happens. So our weekends and school holidays tend to be hectic and the weekdays are much quieter. You just find the social rhythm that works for you :-)
My DS is doing ok at school, have been sending him for social skills / experience as he is naturally shy/ introverted. Was so tempted by HE for this year (reception) but wasn't brave enough as worried he wouldn't have had enough socialising with kids his age as they would all be in school and we live on an island with only a small handful of HE families, and huge population of school children, so not much opportunity there.
Would HE'ers recommend HE with a shy boy and no significant HE group activity?