As Mara said, start short. Take one step away, do only 1 or 2 seconds if that is all Remy can handle at this stage. Do keep him on leash if this will help.
If he doesn't get what "stay" means, start with a down, and crouch down to hold him down while you do a Oh-so-graceful duck waddle, step away, wait 1 second, step back (all this still crouching and holding him down with one hand), release with a treat (okay, now you can stand!) and play and party time, whoohoo!!! Gradually you can just bend over and hold your hand out over his back to block him if he tries to get up.
DON'T RUSH THIS! Stay at one or two or five seconds until it is solid! Do this for two or three weeks if neccessary before going longer. If you find that Remy just keeps boucing up, or coming to you, you are leaving him for too long. He isn't ready. Go back and work on shorter intervals.
At 7 months, he still has a very short attention span. Practice 2 or three times a day, for a few repetitions each time. Work up to 1 minute (both in a sit position and a down position), work around distractions and different situations (for example, he always bounces out of the car and starts running for his toys on the lawn? Nothing wrong with that, but once in a while, make him do a down-stay of a few seconds when he comes out of the car, then release and let him get his toys, just good mental discipline and training to mix things up and remind him that he lives and breathes by your say so!
You should work up to 5 or 10 minute down-stays and longer, but do it gradually, over a period of MONTHS!