If he is in fact intolerant to coumadin then you either need a repeat operation to place a bioprosthetic valve or you need to address systematically what the side effects of the coumadin are: is he bleeding if so from where? If other side effects these can be addresses in multiple specialists.
If you are only been seen in a coumadin clinic, then see a hematologist. Its likely you can do heparin while you consider a transition to another med. Lovenox is another blood thinner. Both need close monitoring just like coumadin.
Not sure what side effects he is having?
Lovenox is a general blood level that needs to be achieved and is injected 12hrly and is pre packaged so no drawing up to do. Heparin is more like coumadin in terms of how much monitoring it needs and the optimum level needed which changes sometimes frequently, it is also a 12hrly injectable and need to be drawn up to the correct dose for each shot. The advantage of coumadin is the oral tabs.
Either way a hematologist and cardiologist together can make a good decision in terms of how to manage what is happening.
I had a trio which also incl my PCP, they still talk to each other and manage what happens with me. My biggest issue while on coumadin was a bleeding inner ear, which lasted a few days, then after it clotted, took a while to clear.
Hope you get some resolution soon. Best wishes.
Fiona