Speech is not the same as language. Both receptive and expressive language skills are important. Your child can have no trouble with his speech, but have language deficits.
You need to get your son to a speech therapist for an evaluation. This way you can get a plan together to help him. It is extremely important for you to do this because he will not succeed in school if he isn't able to organize his brain to properly communicate.
There is a lot that you can do at home to help him, but you need to work WITH the therapist in order to do it. Therapy is just a part of it. The home program is what is most important. But you can't do it on your own. Witnessing his sessions with the speech therapist will help you. Take notes and go by her home program. Do it EVERY DAY, without fail. Even if your child doesn't like it. It's hard because the kids have to THINK. Even when it's "fun", they have to think. But the benefits are huge. When critical thinking skills are mandatory for higher learning in school, not having them will be SO hard on him.
My son had very poor speech. That was our first stumbling block. As he progressed, it became apparent that his receptive language skills were an issue. Working on receptive skills helped his expressive skills. It took a LOT of work.
Early intervention is SO important. You seem to be behind with this. Get working on this now.