You probably need to start with a good dental exam and deworming. Floats must have extensions on them to reach the back molars and drafts tend to be extreemly sensitive to sedation so make sure you run these past your vet before you start.
Without knowing what your cups and scoops are, you're probably giving him enough to start. You can add 1/2 C oil to each meal, making it more of a EPMS diet and bump his caloric intake 6 Mcals. Adding a pre/probiotic never hurts. I use Diamond V prebiotic. It's cheap ($36/50#) and does a good job.