I had a few thoughts of things that might help...everyone is so different so it can take quite a bit of experimenting to find something that will work. But, keep trying!
Marion has a good idea, using a light sedative, an anti anxiety medication such as Ativan (Lorazepam) can often be helpful. It can be given orally or even dissolved under the tongue if needed.
Compazine is in a class of drugs called "major tranquilizers" and it can work well, but can also be quite sedating, especially in older people. Zofran is well worth trying...it's usually taken 3 times a day. Another drug in the same class of drugs that may work better than Zofran is Kytril, usually taken 2 times a day. They both have few side effects as well and each is now available as generics, so that can help a great deal with cost. Prior to becoming generic, many insurance companies refused to cover them as they are quite pricey.
Another drug that may be helpful is Reglan (Metoclopramide). It is taken 4 times daily, usually 30 minutes before meals and at bedtime. It allows the stomach to empty more quickly as well and can often be useful when combined with the other anti nausea medications.
An important point is that any of these work best if one can prevent the nausea rather than trying to get rid of it. Taking the medication regime on a regular schedule is often especially helpful in someone like your step dad who has such chronic problems with nausea. Certainly narcotics can also contribute to the nausea so changing to a different one, the MS Contin, is a good idea. Also, all narcotics are quite constipating so make sure he's taking something to counteract that, as that also can contribute to nausea.
Just a few ideas that might be worth discussing with the Hospice people and getting their thoughts.
It is very good that you can be there as an advocate for your step dad and your mom. It's so good they have you there!
Wishing the best to you and your family.