berry:

It's about your only option at this point. If nothing else is wrong with the tree besides sunburn, the grafted part should resprout.

I've had this happen to several of my pawpaws this year - healthy 7 foot trees that started the year vigorously and then just died back with new growth coming in below the graft. This is not a sun exposure problem - a grafted pawpaw having 2 years in the garden should be able to handle lots of sun; I'm suspecting some type of delayed incompatibility issue. As Fruitnut suggests, you can remove the growth beneath the graft and see what happens, but unfortunately, I think your tree has rejected its graft and probably needs to be replaced. I would not consider this a good candidate rootstock for regrafting given its history.