Three months is actually far too fast for your son, I have an autistic son who is very overwhelmed by change. For introducing a partner I would start by talking about (p) when you are with ds, (p) is Mummy's friend, p likes such and such, oh that's funny p was telling Mummy this story about x. When ds seems comfortable with hearing you talk about p, you can start showing ds photos of p, 'Mummy was on Facebook and found the funniest picture of p, here look', remember Mummy went out last week, here are the photos (include photos of p). Think of a special interest of your son, be it lego, dinosaurs, science, computer games, first talk about how p likes this topic, then when you are absolutely sure that your p isn't a short term acquaintance, you can look at your son being around whilst p visits with (new Lego set, dinosaur toy?,book on favourite subject). Before inviting p over,you can write a social story about p 'this is p, p is Mummy's friend. P is kind a n d knows lots/ is interested in/ lego/computers and football.Sometimes Mummy goes out with p, that's because they are friends. Sometimes P will come to our house to see Mummy, just like I go to Ben's house, that's what friends do. On Monday p is coming to our house to visit Mummy, if I feel overwhelmed I can go to my room and play with my sensory toys. P knows that I find new things tricky, P understands and wants to be ds friend too.
When organising visiting, concentrate on introducing p as a friend of Mummies. If ds struggles with p being there, you can bring out the Lego/ dinosaur toys/ computer games and p can broach his interest in Lego and how you (mum) have mentioned that ds likes lego, he saw this and thought ds might like it. Hopefully with the prep work your ds will feel safer and happier for be in your partners company.
I would recommend thinking of somewhere your son can retreat to.somewhere that feels safe/ a calming activity that your son enjoys as the environment of the meeting. If your partner loves/ cares for you he will be happy to wait and gently introduce himself.