Hay, sorry to hear about your husband. Thought I would just put in my 2 cents...
HO can turn into a very chronic and life dibilitating condition if not properly treated. Bone scans are essential in assessing the maturity of the HO, and invasive treatments should only be under taken once it is sure to have reached this stage.
HO can also be provoked further by local irritation and inflammation. I once had a client with HO who's condition became severely life affecting due to constant irritation (man refused to stop certain ADL's). This is sometimes what makes HO such a large problem in people with TBI's. Therefore, it is probably a good idea to talk to your therapist about exercises / stretches which cause the least irritation to the bony mass.
In your situation, I would double check with your MD to ensure that it is the appropriate time to undertake surgery. If the HO has not reached maturity, continue with the therapy until it has and then assess the ROM / functional limitations and decide on further treatment from that point.
Far from everything is known about HO, and this information is just off the back of my skull, so please, as allways, consult your doctor before acting on any information providedThanks!