I will start by saying I think you need a second opinion. It is definitely not normal for a 5 year old to have such limited understanding of what others are saying, or be unable to use sentences.
Conversational skills are something children this age may need to work on, but if your son can't follow simple instructions of several steps or respond appropriately to questions and carry on simple back-and-forth conversation, he is probably significantly behind on his language milestones and should be assessed for medical or developmental problems. (Note that I don't mean that it's a problem if he won't perform these behaviors on command every time- But if he's actually unable to.)
If it is found that he does have a speech delay or similar issue, then you should receive a treatment plan and/or therapy from his medical professionals, and following that plan faithfully is the best route.
If you are unable to get another assessment for your son, or no problems are found, then you can work on his speaking and language skills at home. These are some easy, classic ways to practice these milestones:
- Read to your child every day.
- Ask him open ended questions during play: Where is the car going? What does teddy bear want to eat for lunch?
- Ask him to tell you a story, or retell a favorite story or movie
- Talk to him about his interests, and really listen to what he says
- Say nursery rhymes together, or sing songs.
- Praise him when he makes a good attempt to communicate.
- Talk about his feelings, and the feelings of others. "You seem really sad that it's time to leave the park." "Sister is so happy because you shared with her!" "Are you excited to see grandma?" "I'm frustrated because I can't find my keys."
- Correct speech mistakes without shame. (If you child says, "I throwed the ball." Respond using the word correctly, but without a criticism: "Wow, you threw it really far!")
For more info on helping small children develop language skills: https://childmind.org/article/helping-toddlers-expand-their-language-skills/
In toddlers, speech and language delays are closely associated with tantrums and aggressive behavior. A child that can't make themselves understood is likely to lash out in frustration, or just take things that they don't know how to ask for, or have trouble appropriately expressing emotions. Working on language may help with aggressive behavior, but in the meantime, firm but fair boundaries of behavior and how to treat others are important. Depending on your parenting style, you may use timeouts or similar consequences for unkind behavior towards other siblings. This will likely go better if you also work on learning to deal with his feeling in a healthy way. You can try things like punching a pillow or jumping on trampoline, deep breathing, going to a cozy spot to 'cool down' when he needs to, and using words to let others know how he feels.
This article has been useful for me in understanding how to help my toddler with his strong emotions: