The biggie for Birmans is the feet markings. Look at this site. There are other things but bad feet markings, as I say, is the biggie.

I show my Birman but have only done so for one show. Birmans, apparently, are one of the more difficult to show as the markings are so specific & the grooming is only second to Persians in being particular & time consuming. It may be worth going with your boy as an entry so that you can meet others who know about Birmans and/or to get an official point of view. If you don't get placed on the day, look out for the judge & ask them about your boy otherwise you won't know as those placed get reports on cats after the show.