Hi AussieMom!
We started at birth, but Zed has GERD, so not much happened after the first week, until we finally got his reflux under control at 6 weeks.
We never stopped letting him hang from our thumbs. And we did the minimum the doctor recommended for tummy time as well. Which is 5 minutes per week of age, per day. So a 17 week old should be doing an hour and a quarter (1:15) of tummy time per day, for example.
After his meds were working, we started with the inclined track. It was the short-side of a double bed long (which I don't remember the size of at the moment) and it was raised just under 7" at the top. We put him in it 15 times per day (because he was "behind" physically) plus he was on the floor unless I needed to do something in another room (than the living room), we were out, he was asleep, or a grandparent refused to set him down. Outside or doing chores I wore him in a baby carrier that kept him upright (If you get one I recommend either a Beco Butterfly or an Ergo. The Baby Bjorn outward-facing ones are bad for both of your backs.) so he was still strengthening his neck.
Otherwise we did the HSIYB (How smart is your baby? by Glenn Doman and his daughter Janet) program to the letter. Plus we do Itsy Bitsy Yoga, and we sign with him. (And EC, which isn't really relevant)
He is 9 months old now, just started running on Thursday (eep!). He can swing back and forth on the monkey bars, but isn't going forward yet. He says more than 10 words, Off the top of my head: Mom, Dada, Gamma (Grandma), Gampapa (Grandpa), at (cat), dog, up, now (down), bye bye, og (frog), nkgoo (like saying the nk-you part of thank you). He signs potty, bird, more, milk, cracker, please, help... and more I'm sure I'm forgetting. He has a wide variety of food words and signs that I can't even think of right now.
Anyway. My point is not to brag, well MOSTLY not to brag
, just to show the effectiveness of the program. From birth, or near birth, it does amazing things.
The balance activities seem to be the biggest, active, thing you can do to help your baby. Floor time is important, but you can't dictate what they will do once they're on the floor. With the balance activities you know your baby is getting the benefits. Once you have a track built (A piece of plywood covered in linoleum would work in a pinch, it doesn't need to be padded if you aren't using it for sleep IMO) an incline track helps a lot.
We always focused on where he was uneven. So we'd look at the chart and say "Well, today he can do everything in green, and 5 of the 6 blue, so let's work on getting him to say couplets" which is where we are right now.
For you and your son, getting him to hold things would be a good next step, because you know his arms are stronger than his hands. A 3/8" dowel would work, or maybe smaller depending on how big his hands are. Just get him to hold it (with both hands if you can) and then pull gently, he should pull back. This will strengthen his hands, just pull progressively harder until he can hang from it. It might take a few days, or a few weeks, but you should see/feel progress.
Doman suggests doing things 10x per day, or 15x remedially. It's up to you if you think his hand strength needs remedy, or just the standard program.
Then the inclined track (10 or 15x per day) and the balance routine (just once or twice) and you should be on track physically.
HTH!