Here is my interpretation/summary of the what OP meant:

"I think my snake is a little on the thin side. I'd like to know what the average weight of a 3 month old, 17-18 inch BP is. Also he may have eaten a few pieces of fur bark. How likely is it that he will get impacted?"

From what I've seen posted, the growth pattern of BPs varies widely. So there isn't any one "right weight" for a BP of a particular age. Some people who have had many more BPs than I may be able to give you a better idea of an average. You could post pics of your BP and ask people for their opinions on whether or not he looks thin.

If you decide he is too thin, you could either bump his prey size up a little bit, or many people feed young BPs every 5 days so you could feed him more often if you aren't doing that already.

I'm not sure what fur bark is... is it like repti bark? How likely he is to get impacted depends at least partly on how much he swallowed and you make no mention of how big the pieces were that you think he may have swallowed. I have not used that kind of substrate, but from what I've read of other people's experiences, occasional small bits of substrate don't seem to cause problems.