In glaucoma my eye pressure has dropped from 28 to 15 using travatan. Is it likely progression of glaucoma and nerve damage has been stopped?

Slowed. Maybe not stopped but much better protection with that result. This condition may always progress despite a nice response to the eyedrop, but it is the rate of progression that will in most cases slow considerably when the pressures are well-controlled. Different stages of glaucoma will require different therapeutic approaches, from eye drops to surgery. Therefore it is critical to be monitored.
Yes. That is a wonderful response to the medication. If you keep your pressure that low, it most likely will stop the progression of the nerve damage. You have to keep checking it though as it may change in the future.
No, but. Your iop reduction is excellent. The progression of your glaucoma has been markedly curtailed. Cure is not possible at this point in our present understanding of the pathophysiology of the disease. But one thing we do know is that lowering the pressure has substantial sight preserving benefit. Your iop has to be monitored for life to reassure the pressure reduction is adequate n.