That's great that he seems to be able to control it so well. My own current opinion of TMAU (if it's solely TMA) is that I would expect it to be very well controlled for most people. There's a study a while back where they said 85%(?) could stay smell-free (if I recall correct) http://www.ncbi.nlm.nih.gov/pubmed/15043988.
There's a video on youtube where a young boy seems to have 'classic' TMAU and his mum can smell him (which seems unusual for most of us). I don't think he was too concerned about the diet currently (being about 8), even enjoying his fish supper on a Friday :)