It really comes down to the child. My DD (who is now 14) enjoyed BTMRR when she was 8, but she has become a huge coaster fan - the bigger the better for her! My younger DD is not into coasters and was too scared to do BTMRR last month when she was 7.
I advise you to try your child on the Barnstormer first, perhaps 7DMT second, and build up to BTMRR. Otherwise, if you try BTMRR first and he does have a bad experience, it might ruin his chances of going on the smaller coasters and enjoying them.