Hi -
Glad things are moving for him - Yes, it is a fantastic instrument!
Has he stopped piano, or only got to Grade 1 so far? (not that it matters either way.)
Has he done any recorder? The fingering for saxes is virtually the same as recorder, so a bit of recorder knowledge can be a help.
Our DS started alto sax aged 10, having done a few years of piano, and a year of trombone (which he only did as we had been told he was too young for sax, which turned out not to be true!)
We bought a Yamaha student Alto, through the local secondary school, as that got rid of VAT. He had lessons through school, and when he went to grammar school he went straight into their jazz big band, which also played for their musical show every year. He took several Grades, but you DON'T have to do every one, and can skip a few here and there. To do the higher Grades you need to have Grade 5 Theory, so keeping on with piano is an advantage. Aged around 12 he got into the county jazz band, which played concerts around the county. Eventually he got Grade 8 Distinction, and also Music 'A' level (that needs a lot of theory, and the ability to compose bits in the style of Mozart, Bach or whoever.)
You need to consider whether you want Alto sax (E-flat), or Tenor (B-flat) and is considerably larger. (There are also the smaller Soprano, and much larger Baritone [Lisa in 'Simpsons'], but they are not usually for beginners.)
Associated Board are the 'standard' Grades, the alternative is Trinity; was 'Guildhall Trinity' but I think the Guildhall bit has been dropped.
Once he can play a bit, there is a brilliant library of a hundred jazz improvisation books with CDs, to teach scales, theory, and all the famous jazz sax soloists. This should link to them HERE