Teach your puppy to crawl - that is, move forward from the 'Lay' position. You can use this trick to let him crawl under a chair, for example, or - as shown in the picture - under a vertical bar.
How to do it
Starting from the 'Lay' position, say 'Crawl', hold a treat in front of your dog's nose and move it a few inches away from him: if he stands up just say 'No, CRAWL!', and start over (make him 'Lay' again). Praise him a lot and give him the treat as soon as he crawls, even a few inches.