How can I get my baby boy off the breast n get him on the similac?

Depends on his age. Neurotypical 5-month-olds have procedural memory & 6-month-olds have learned behavior. Introducing the bottle then may require dad's giving it to him at first. Breast milk till 1 yr.(then weaning to a cup is best, but if you must switch, hold him with his head up at least 30 degrees for all bottle-feeds & minimize air-swallowing. Wipe out his mouth after the last feed before nap or bedtime.
I won't recommened. You can replace breast milk feed with the same amount of similac with iron 20 cal bottle feed, when your milk is not available. Frankly, unless it is absolutely necessary i won't recommend.