I live in Austin, but the ARC of central Texas or Austin/Travis County MHMR is where one would start here to get resources and help for a person with special needs. If your grandson is not in school, get him to the nearest elementary school and have him evaluated for services. He will qualify for preschool as well as all the therapies that may help him (OT, PT, speech, etc.) and you will suddenly have a whole team of educators working with you and giving you ideas for his appropriate development.
My son has special needs and started school at age 3, after going through the ECI program in our area which he started at 9 months old. We have been so lucky to have had such wonderful therapists and now teachers to help me help my son be the best he can be.
My next advice to you is that once you get started with what help is best for your grandson now, to stop reading about what milestones he "should" be reaching and just focus on his own personal growth. It's very hard to watch a child struggle, or just be slow and steady, but looking at what he is not will only depress you. Or at least it did me, so now it's not about my child "catching up", but just doing what is best for him with every effort. My son has Opitz syndrome with moderate MR and is turning 6 soon and learned to say "Santa", "Merry Christmas", and "cookie" this year in time for the holidays. Such a charmer he is! :)
Enjoy your sweet boy! Best wishes.