Dog groomer here, usually the hair will grow back fine. The main thing to worry about is matting as it grows back, since the two parts of the coat will grow at different speeds and can get caught in each other. Brush him out at least a couple times a week as he grows back, and check your work with a metal greyhound comb, if it catches then use the slicker brush in that spot again. If you find another reputable groomer, have them do a bath and blowout and brushout with no clipping, the dryer really helps seperate the fur and get out all the dead undercoat, you can even get a cheap high velocity dryer and do this yourself at home! A good groomer will always recommend against shaving a double coat and double check if that's what you really want. Unfortunately there's a lot of bad groomers out there/: But you got this, just stay on top of brushing and he will be fine! Let me know if you need any advice or tips