I can't tell you if this would relate entirely to a younger/smaller snake but my girl has just gone 9 and a half weeks with nothing but a couple of small urates. I don't think it's too uncommon for royals to take their time over it
I kept her on the same feed schedule throughout (shes just hit 800 grams and is on large weaners currently) and I'd say 3 days ago she finally dropped the mother load
personally I don't think you're underfeeding necessarily - and I'd say that the fact there is weight gain kind of proves that. The prey might be a little on the "small side" which could I suppose result in less frequent poos simply because there's not enough there to justify it, and most of the food is being absorded and/or thrown out in urates
If you handle him regularly then when he's out try giving him 5 to 10 minutes of slightly pressured stroking from head to tail starting about half way down the body. It might just help to relax his muscles and loosen up anything that may be slightly hardened
I'd wait and see what happens over the next 4 weeks, but if you're in doubt and getting worried take him to a vet for a quick check up. No harm ever came from a vet visit, even if it just puts your mind at rest