1. Speak with the pediatrician.
2. Have his hearing checked.
3. Buy "picture dictionary" books and look at them each day or night before bed with him. Do not specifically expect him to repeat the words, but if he can get excited and praise him.
4. Name and label things in your home or anywhere you happen to be.
5. Give him choices in his life and enunciate VERY CLEARLY:
Do you want the RED cup or the BLUE cup?
Look at that RED truck.
Let's go to the store.
6. Engage in play with him and talk to him in ways he can repeat.
Be sure you are speaking to your son in manageable sentences. Children do not learn to speak simply by listening to normal adult conversation, but in having the adult model "bite-sized" phrases for them to mimic. This will likely do as much as speech therapy, although if you are gone too much of the day then it might help, as well as help you model your communications with him.
In preschool, the hope is that he won't feel left out, if he is not communicating. Watch him and if he doesn't seem to behave well or doesn't want to go too many times, either reconsider, or observe him from a private area, or ask the preschool manager how he is doing. Be sure the preschool knows he is not fully communicating. They may also have some ideas in how to help him gain more ability to communicate.
How do you know what he wants for a snack? For lunch? Does he communicate in any manner? Give him choices and in some way expect a response. If he is simply pointing, then say the words (red cup/blue cup, etc) very clearly. Hold the cup up: "Red cup, nice red cup. Ethan wants the red cup". Or similar communications until he listens well enough to repeat. I suppose the speech therapy will do many of these things and others, and then help you to do this at home, as well.
Do not just let your son allow everyone else to do the communicating, and do things for him. This may be happening especially if he has older siblings and perhaps he is a quieter child. Give him plenty of opportunities to mimic, communicate, and express himself and be sure you are not just giving him what he needs without, at least sometimes, asking from you in some way.
He may be a quiet child who doesn't have many needs - at this point. A lot of children do not understand, "no" because they don't want to. He needs tools. Give those to him.