Have you tried asking his teacher who he plays with, who seems to really like him, who else might be a bit lonely and up for friendship, what club or activity is best attended?
The sad truth is that many parents will become friends from chatting at the school gates, from bumping into each other at after school clubs, from helping out on trips, providing reciprocal after school childcare/play dates. Sometimes these arrangements are made spontaneously, on the same day, as the kids walk out of class together.
You are unable to do that because you work full time. There are a great many parents in that same position. But it does not mean that the other parents or kids are being unkind (and your comment about ignoring messages does suggest some chippiness). It just means that you are 'out of sight, out of mind' and have to work a little bit harder.
A scattergun approach of random play dates is unlikely to work because you might be inviting kids who already have a tight group of mates and their mums cba with even more commitments.
Popping up in school holidays after being absent all term is unlikely to work - and I'm not suggesting you are doing this btw.
The trick imo is to identify who he plays with and invite them regularly. Do the fun things that other parents don't want to do - several friends at once, an exciting day out, a little party to celebrate the start of the holidays.
Keep doing it even if it isn't initially reciprocated, because your ds is enjoying himself and strengthening friendships through shared experiences.Chat to the other parents when they pick their kid up from your house, so that it's not weird to ask whether they fancy a park trip in the holidays.