First of all, please stop feeding your snake through a syringe.
From what I can tell, your snake ate live at least once. If you are that freaked out about the weight, continue to feed live a few more times, not only to put on weight, but to get it into the habit of feeding again. You can try F/T later when you feel better about your snake's weight, or when it outgrows hoppers and pups.
Downsize the enclosure. You don't have to get anything permanent- most stores have plastic tubs for pretty cheap, and all you have to do is put some airholes in the sides and get some clips/straps for the lid. Once your snake is established and feeding consistently, you can switch it back. If that is not an option, make sure you have multiple matching hides, and stuff the current enclosure with branches/chunks of wood (baked first). BPs love spaces we would find claustrophobic.
If you feel comfortable posting your location, maybe there is someone near you who would be willing to take a look at your setup/your snake, and help get him back on track.
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