I've been on topiramate (Topamax) for around 8 years and haven't had any problems with weight loss. This can be a side effect, but it's usually minimal. The dosages of this that are used for migraine prevention are typically fairly low compared to what they use for seizure treatment. I'd expect that MS Contin would cause more weight loss difficulties, especially when used in the long-term as this is being used. But only your dh's doctor could decide this.
Do none of the typical migraine medications work for your dh? These wouldn't likely result in weight loss, but not all migraine meds work well for all people. When all else fails, narcotics are prescribed. If this is the case, then he might have to find other ways to deal with his increasing problems with weight loss. And he'd, again, be best to check with his doctor about what methods would be best, based on his medical condition and other factors.
Forgot to add: If your dh has severe migraines, then he likely won't want to get off of these medications. This becomes a quality of life issue. I tried going off of topiramate lately and had only decreased the meds by one quarter of their strength when the migraines began to hit me hard and repeatedly. I decided that I'd rather live with being on these meds, and paying for them, than to ever have to put up with that degree of pain again and for the rest of my life. Likely your dh would feel the same. True, severe, chronic migraines are not something that anyone wants to just try to cope with on a daily basis. And it sounds like that's what your dh has. Don't pressure him to go off of these meds. Find a better solution, or find some other way for him to maintain his weight. :-)
"Why fit in when you were born to stand out?" Dr. Seuss
"Your beliefs don't make you a better person, your behaviour does." (unknown)
"If everyone is thinking alike, then somebody isn't thinking." George S. Patton
The poster formerly known as Writer Mom (stolen from Prince)