I’ve been trying some of these ideas offered above, and I’ve seen some progress today. When I pretend not to understand, he will now say “milk” or “coo” (seems to mean cookie, or snack, to him). When I got our coats out of the closet, he actually said, “Mamama dadada baby go.” Ok, a few extra syllables, but I know he understands, at least. Actual words! And I suspect that’s an early attempt at a sentence!
He was around a couple of older kids today, and responded to them in a great way. He wants to play with the big kids, it seems. So I’m looking into a playgroup, but we’re sans wheels, so it’s difficult to travel much. I’m trying to find one close to here (SW 'burb of Chicago). I’m thinking of one with kids about 2-4 years old, ideally, so he’ll have someone to emulate (and chase).
Just a wild idea, but since he tends to hold himself up to “walk” using his toys and various furniture, I tried moving his things a little further away from each other. Now he’s taking 3-4 steps on his own to get to the next hand-hold.
I plan to have a gentle talk with Grandma soon, to tell her even though she was SuperMom who can achieve wonders with 3 toddlers and a baby at the same time, I’m taking a less demanding approach with my one-and-only baby. I’ll be diplomatic, though: “And I would rather nobody pressures him too much, but thanks for your concern, Mom.”
BTW, I perused the links regarding developmental difficulties and disorders, and I am greatly relieved; GypsyBaby doesn’t seem to meet the criteria for any disorders or anything. Whew, what a load off my mind!
I want to thank you all for taking the time to offer your parenting expertise to a first-time mom. I’m getting better advice here than on all the parenting sites I frequent! 
