I've worked w/ lots of infants and toddlers as well as adults. There are simple things you can do to "play" but are actually engaging muscles that are required FOR crawling, walking, etc. Do you let him crawl lots WITHOUT shoes? These muscles in the feet and lower extremities are difficult to engage when encoumbered by SHOES. All the cute little boots, shoes, etc ARE cute, but MANY times restrictive and NOT comfortable for the child.
I showed this to someone just this week in the gym. The toddler was struggling to crawl. I took her boots off and off she went! Not extremely efficiently, of course, but she WAS able to get there. She'll get better w/ practice, but the shoes were holding her back.
If children/toddlers don't develop the correct muscles and "in the right order" it's hard for them to do other things. you DON'T want them to stand and walk before they learn to crawl WELL.
First ? I need to ask.......how long has he been crawling? Was he slow in getting there? How WELL does he crawl? Does he crawl w/ R hand and L knee or R hand and R knee? You may think this makes no difference, but it CERTAINLY DOES! Your gait pattern is R arm and L leg as you swing FWD. This has to be developed FIRST using shorter levers and closer to the ground for stability.
Let me know and then we can go from there. It doesnt' take long once you engage the right muscles. In addition, once they feel the "freedom" of being able to do this......they just take off from there.