Hi A.,
As a former preschool special ed teacher, I would suggest that you start with simple sign language to encourage your son. Teach him a few each day that will help him to learn the word sign attachment. You can find the signs on the internet or at your local library where they will have some books on how to teach your toddler sign language. Make sure you always say the word when you use the sign and help him to make the sign by forming his hand.
In the meantime, you can take him for a speech evaluation. The public school system offers this at no cost to you. They will be able to do a complete evaluation and let you know if he does need the therapy or not. Sometimes, as you said, he could be a late talker just like his brother. Boys don't talk as early as girls. We usually had more boys our program because of language delays. The speech therapist will also be able to tell you about other suggestions to getting him to respond.
I hope this helps. Best to you!
M.