Hi Jack 66.
I can remember my mum going through a similar thing when I left home. She tried to shield me from it but it was obvious. She had my dad at home, too, but still felt bereft.
Is your DS moving a long way away? If he isn't moving too far away, you both might like to meet up for lunch once a week? If he is moving far away, maybe you could go and visit?
This would no way compensates for missing him daily, but at least you wouldn't be faced with long stretches of weeks/months on end without seeing him.
He will come home, too. Make sure he knows that his friends are always welcome to come back home with him.
If you're not working full time, and it's not too expensive, maybe you could start a day/evening course...dressmaking/baking/learning a language or something like that.
Sometimes friendships just evolve, if the emphasis is on learning a new skill, rather than making new friends, it might feel less stressful.
Might it seem less daunting to start something before September? if you leave it till he goes, you might find it even more difficult, I would, anyway.
There are lots of things you could do, but I'm not sure how much time you have to spare. Eg., could you fit in a morning in a charity shop? Is there any other voluntary work you would like to try? If you aren't working full time, what about getting a dog? That would give you the purpose to get up and get out a couple of times a day, and it would be company.
Again, anything that you can put in place now, I would, rather than waiting till September. You've got several months in order to have created a whole new routine and set of interests for yourself.
Doing this might be quite reassuring for your son, too, I'm sure he's worried about you, I was worried about my mum.