Hello. I have an autistic daughter. She is older than your son but still a 'little one' so my experience can help. I have two Havs. Both of them were chosen by the breeders with my daughter needs in mind, so that helped a lot.
She is not too happy when she sees other dogs but she LOVES our dogs. I didn't force her to pet them, everything was as she wanted. I always taught her how to be gentle with them, how to touch them and how to respect their space when they were resting or eating. I never let them together without me supervising.
Remember that when the puppy gets home he will be playing hard and biting a lot (just playing ) I tried not to let the puppies play like that with her so she didn't get afraid of them.
Try to let the puppy visits his bedroom with you a lot so they get used to his life, his voice, his laugh and his possible tantrums also. When my daughter gets angry, it is very natural to them thanks to that exposure.
It can be very difficult during training because of all the attention the puppy needs and your son too. The expen will be very helpful so when you can't watch the puppy he is in a safe place. In my case, I used a small crate a lot. My first Hav cried a lot when puppy if I was out of her sight. It made my daughter anxious so I was moving all around the house with the puppy in the crate by my side. When I was busy with my daughter, the puppy was relax with us.
At the beginning, let your son give treats to the puppy so the puppy knows good things come from his new friend. Involve your son in the training when you practice at home what you learn in puppy school. My daughter said she was a good trainer and was very proud of it. haha Let them play for short periods of time. Remember the puppy needs a lot of rest.
Your son will LOVE having a dog and Havanese dogs are sweet, playful and great companions so it will be wonderful for your son.
I hope this helps. If you have any questions, feel free to send me a message. I will be happy to help. Congratulations on the puppy and welcome to the forum!!! Ahh we will be waiting for pictures!!!