I think this is up to you if the coat is too much to comb or not, not your groomers. If you like it longer and enjoy grooming it, there is no problem with keeping it long. An idea....Instead of cutting the coat short, you can have him scissor trimmed just to shape the coat but only take off a small amount. But, if you find that grooming him is getting harder and want to have easier grooming, then let the groomer give him a full clipping.
I don't think the dog needs to "get used to" clippers on his whole body if he is getting his face/feet/tail done at each grooming. If the dog is used to the clippers in these areas, doing the body will not stress him out anymore than the clipping he gets now.
Personally, I would do a slight scissor trim to shape the coat a little but not cut him short till he is older, he is still young. You definitely may want to have him cut short when he goes into his coat change, I can attest to the fact that taking care of a long coat during the coat change is challenging.
