They have steak at Feathers for sure. I usually do not eat fish either but get your Dad to try the snapper sandwich at the beach grill. It changed my mind.
As for larger portions, there is no issue if he orders two entrees
I would think an all-inclusive resort would be the perfect option for him. If he doesn't like something he orders, he can just order something else. Feathers probably has the best steak at the resort, but we certainly enjoyed the steak at Patois on this past trip.
Dad can order multiple entrees, or a few of whatever. There's always pasta and salad bar at the Palms. Three buffets at dinner per week (including the beach party). He can get hamburgers, hot dogs etc at the Beach Grill. We've learned where to order multiples, ie appetizers at Lemongrass.
This is NOT a meat and potatoes resort. I would highly recommend having dad explore his horizons and taste the local dishes!! He will be happy!