Pitt but aren't Pitts and Rotties about the same when it comes to temperament and training? If you have experience with both that would help. Thanks.
No, they have pretty drastically different temperaments. Some times pit bulls with behavioral issues can be pretty tough to train/handle, because they're terriers, hence very tenacious, but many of them are very easy for a first time owner to handle - especially one that's already an adult. They are naturally extremely fond of all people, but can have issues with aggression towards other animals. However, I know many pit bulls who live happily and comfortably with other dogs, it's just important you pick the right dog (it's a misconception a puppy raised with other dogs will do well around them, it's common for puppies to develop a prey drive or dog aggression during sexual maturity. You're better off getting an adult dog), and be aware of signs of aggression, such as obsessing over other animals (even playfully) or looking uncomfortable around them.
Rottweilers are typically naturally wary of strangers, and extremely protective of their families and homes. While a well-bred rottie who is raised by someone experienced can be taught to tell apart a real threat from something innocent, a less stable dog or one raised by someone without knowledge of animal behavior is likely to become difficult to handle around certain situations. Especially if your son's autism comes with a lot of anxiety on his part, or your part, as the dog bonds to your child, he may become upset or aggressive towards something inappropriate - such as you, or other adults trying to help your son. If you do decide to get a rottie, I strongly, strongly recommend consulting a trainer and going through either a good breeder or an experienced rottie rescue who will offer support to you about training the dog.
Both breeds LOVE children, and are very intelligent. They're both energetic breeds, but I think typically as adults rotties need a little more exercise. Each dog is obviously an individual, and it depends on their breeding, but as a general rule a pit bull is going to range from "love everyone no matter what" temperament to a watch dog that barks at intruders/trouble, and a rottie is going to be more likely to try to apprehend a threat (and how well that dog can tell a real threat from a child's uncle teasing them or a guy in a scary uniform is going to depend on their socialization)
People often think getting a puppy will allow them to train and socialize the dog the way they want. Unless you're very knowledgable about dog behavior and training, this isn't the case. Puppies temperaments change as they grow and develop, and they're very impressionable, meaning even a mildly bad experience can cause behavior problems for the rest of their life. With an adult dog, you know what you're getting, they will bond just as closely to you/your family as a puppy would, and there is less of a chance of the way you train and act around them causing behavioral problems.
If you do go with a puppy, make sure it's from a good breeder (definitely DO NOT go to a pet store. You can research how to know a responsible breeder from an irresponsible one, but the basics would be that they health test their dogs, you can meet the dog's parent's, they provide early socialization for puppies, and they'll offer you support and help throughout the dog's life). I'd also recommend talking to a positive-reinforcement based dog trainer, to help make the dog the best possible pet for your son
If you want your son to be able to meet and interact with different types of dogs and their coats, you might want to look for a pet or breed expo, or all breed dog show, in your area. You could also try going to a local shelter and seeing about meeting some of the dogs there.