My son was like this. What helped was establishing interests that he could share with others. So in ds's case, football stickers/matches (rather than actually playing it although he did that too, from about 9 very badly) Lego (Lego club at school as a communication pastoral group for those with social communication issues) Also soft play, outings on trains. Basically things that gave ds esteem, kept him out and about, and kept him communicating in positive way about things and ideas. We read him lots of stories too, and I'm sure that helped with modelling of conversation and thoughts and empathy. So things like Wind in Willows, David Walliams. Drama and singing and dancing also was a good outlet for his sociability.
Ds was not at all good at conversation or interaction at this age. Very dominating, interrupting wanting others to play by his rules, getting upset in playground at children not playing his way not liking him. He also had a lot of sensory issues we werent aware of, that set him off.
He is much better now, and has friends, gets out and about and is very polite and communicates beautifully, can listen, and exchange views. Doing A levels in a comp. Has empathy and understanding.
It takes time, but build up skills slowly through every day life situations and don't lose heart. The more you can share with him, the better he will be equipped to communicate with others outside the family..so 1:1 with parents helps later social skills.
Out of Synch Child Has Fun has lots of good advice. Ds gained a lot from gross motor skill situations and that helped him socialise too without too much pressure. It is hard, there will be tears (from you) many setbacks but don't lose hope.