I would stick to large mice(25g)every 5 days until hes 200g then move to 1 rat weanling(25-35g) every 7 days. @500g bump them up to 1 small rat(40-75g) per week where they stay.
Ive found with my feeding schedule all my snakes will poop before their next meal and grow at a rate of 50-100g/month. I think if your snakes not pooping its either being fed too little or too much. In your case I'm guessing too little b/c you just got the snake. Its much easier for a breeder to sell a small baby than it is a larger snake. It also costs $$ to feed the babies so its much cheaper to feed them just enough to keep them healthy and alive. He probably needs larger meals more often and thats why hes not pooping. Hes using up every nutrient possible bc he doesnt know when the next meal is coming. Easy fix is to bump up feeding to large mouse every 5 day or the rat equivalent(pup).If you feed it too much it will obviously take longer to digest and extract all the nutrients it needs thus prolonging the dump. following this kind of feeding schedule will eventually lead you to a snake that refuses bc you're over feeding them.
I have to disagree with Starmom as a small rat is rather large for a @200g baby.