Children with autism are each very different but communication tends to be difficult for most of them. Getting a child with autism is speak is usually a great challenge even for a trained speech therapist, so know that there is usually no quick fix.
Observe carefully your child's communication. Encourage eye contact by holding object he desires near your face when you speak. For sign language choose a few words such as "more" and "eat" and help him form the signs by shaping his hand movements each time he would appropriately use the word. Choosing a few words that he can use many times each day give him lots of opportunities to practice and learn them before adding new signs.
For vocalizations, as a speech therapist, I've found that many of my children will imitate a sound in an "echo chamber" which is a large mouthed bucket or bowl. The echo microphones do not work as well because they tend to put their mouth on the device. The large mouthed bowl/bucket is less likely to stimulate the desire to place the rim in the mouth. I begin by copying any sound that the child makes (often a grunt or whine or cry). I hold the echo chamber near my face and match the child's sound EXACTLY or as close as possible. I use the same intensity, length, and pitch used by the child, then immediately turn the bowl toward their face and wait. Sometimes, they will move their lips or appear to be trying to make a sound but cannot. Clap and celebrate their effort and continue the process. As they are able to repeat the sound, then begin to change it up a bit. Stretching out the sound longer, repeating it twice or changing just one sound in it. As their imitation skills improve, continue the process until they are able to repeat words and phrases. The is not an overnight process, but I've seen it work for many, many children. Be sure to include all family members in this new imitation game, and he will be more likely to enjoy it. Make it fun!
I ask parents to imitate every sound their child makes for 5 minutes each day. This helps the parents recognize the variety of sounds a child makes and in what context and gives the child feedback and focuses his attention on his own vocalizations.
Also, communication using pictures is often another helpful strategy to begin with children with autism. Take photos or box covers of his favorite foods, toys, etc. Keep the objects out of his reach and have the photos nearby. Have him begin by reaching for the photo and then he is rewarded with the object. You can put the food photos on the fridge or cabinet and toys on the closet doors. Use no more than 2 choices at the start. Help him select the photo in the beginning and reward him immediately so he makes the connection between the two.
Your speech therapist will help you along the process and give more specific ideas that fit your little one.