I'm sorry to say this, but...90 days ins't that long. Yes, it appears to be a long time to you, but to him...it's just like yesterday.
There are going to be times that you won't hear from him and the more you send messages to him, the more likely he's not going to answer them. You are pursuing him. I know you want him to know about your son, but step back a bit and be still for a while. If you remain still and quiet for a bit and allow him to realize that you aren't calling or sending messages, he just might get curious and contact you. Unless you have an absolute emergency of any kind, be still. Okay?
If you have truly accepted that he's gone, then let him go. I'm not saying divorce him, but leave him be. Live your life as if he's not going to return. Live your life to the fullest and be there for your son. Right now, you are the strong and supportive parent that your son needs. His father, well...he's out to lunch for a while.
Can you pretend that your h is on a trip to Mars and can't contact you for a while? This may help you detach more if you can think in those terms.
Sit quietly, the answers will reveal themselves when you least expect them to. The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.
