Hey nae. I have an American QH and a full blood Clydie and an ex racer as well as 2 ponies! I havnt ridden my boys since falling preg either. I would start ur boy off on the lunge with a roller to put a bit of pressure on his back. I would do that for a week or 2 then lunge with the saddle fir a week or so and then jump on for a quite walk.
Goid luck and enjoy!