I intend to play my new running back in the full back position or as a bruiser, but I want him to gain fb experience but keep his rb weight(I will probably pass and run with him more than blocking). In order to maximize exp at fb position but keep weight off, should I swap his position for camp to gain exp, then swap back to drop weight back in preseason; keep him at rb then swap to fb in preseason then back in regular season; or just keep him where he is at but play him at fb as much as possible?

If you want him to have RB attributes at FB then just keep him s RB and play him at FB. His rating doesnt really matter, it's his attributes that matter