When you switch you want to switch with a rat identical in size to the mouse you were feeding, you do that for 2 to 3 meals and upgrade in size if necessary.
It's about tough love and know when to resume to normal feeding and what works, at his size there is no concern and yes it could take months without causing any issues.
If you have access to dirty mice bedding use that as well.
Feed what work weekly and worry less about switching.Also is there a way I can help him put on a little weight?
A jumbo mouse for a snake that size considering it is a male is more than enough.