Bartholomew by itself is gorgeous - I get a strong mediaeval monk feeling from it (probably because of the
Matthew Bartholomew books, which are set in middle ages Cambridge, although the title character himself is not a monk).
Batt is awful, though, sorry - 'batty' is slang for 'crazy', and I'm sure he'll be called Batty a lot. Plus I just don't like the sound.
Also, parents can only go so far in ensuring that their particular nickname is used. He'd still get called
Bart, and the kids would have no trouble rhyming it.
If you don't mind taking the risk about kids noticing the '
Bart' part of the name, I'd suggest
Bartholomew "
Tolly". It's cute, and it's far less teasable. WDYT?