Hi Donald
Your son has been through a lot so I think it is reasonable to assume there could be a lot of psychological overlay to the problem
The fact that some time during the day he has reasonable mobility would suggest to me that things aren't too dire. The muscle guarding pattern could be due to pain mediated response is the more time his he is up and around the more time stress is on the neck and the more pain he is in so more guarding. This is an unconscious problem. What are his pain levels like as the day goes on? - this may indicate whether this could be or not be the case. If this is the case perhaps he could do some napping or really comfortable lying down during the day even just for half an hour to reduce the pain cycle. Sometimes the application of a warm pack (eg Wheat bag heated in a microwave - but not too hot and wrapped up in a towel, or a good old factioned hot water bottle) while he lies down on it in a very comfortable position can help.His neck mobility is very limited, although we suspect a lot of this is psychological for fear of it hurting more. When he wakes in the morning he has a reasonable amount of motion, but as the day progresses, he gradually pulls his chin in towards his neck and tilts his head slightly to one side.
If the surgeon of physio is really adamant that he needs to do the exercises now and not delay it then what about pursuing the psychological component? A good child psychologist might be able to help him. They can often help your son adopt a change in his thinking about what he has been through and what he is experiencing if anxiety is a big factor - which sounds like it is.
You should discuss any possible adverse effects and side effects. Yes it is an antispastic drug and your son doesn't sound like he has spasticity but your GP is probably right in that he may be able to relax more as it targets muscles to relax. Yes Baclofen does have a potential for a withdrawl syndrome but your son might not need very much and as long as the drug is tapered down slowly this may not be a problem. People often don't like the side effects however.Our GP has just prescribed Baclofen as a muscle relaxant, but we haven't started it yet due to finding out that is mainly used to treat spastic conditions (don't know if we truly have that) and that he would have to be weaned off it again afterwards.
If he is objecting so violently to the physio intervention I am not sure if it is that productive - again if he has been through a lot and if he has reasonable range of motion in the morning and it is only a month - may be the physio could ease off a bit and pick it up as his neck muscle stamina improves. If the physo has good rapport with him and he doesn't mind the massage perhaps emphasising that more might be good. Work on increasing the trust might help. Paediatric physios are often good at turning the exercises into a fun game and that can also help.
Another option is some Feldenkrais or Alexander lessons. If you can find a physio who is good with children and practices one of methods this can be very helpful. A Feldenkrais practitioner is generally very gentle in their approach and communicate that is is safe to move. Hoever I would not go to a lay practitioner - go to someone who understands the medical and surgical problem your son is facing.