LOL if he is going to do it, let him. Believe me, once that baby gets here you are going to wish you could just sit on the couch!!
This comes from an old pro mom of 3. I didn't have horses when I was pregnant unfortunately. I guess if you were doing it all along it wouldn't be bad but I can tell you the 3 weeks before my last child was due I stood outside bending over to pick up pecans. OMG - the next day I could barely straighten up. Being pregnant puts a whole new dimension to everything. Enjoy your horse and your pregnancy and let him doing the heavy lifting for now...enjoy the pampering while you can!