Hi Changebagsandgladrags:
First the link to Mumsnet's advice on progress through National Curriculum levels suggests that notionally the average year 1 child should attain a 1a (in reading/ writing, maths & science) by end Year 1 regardless of age - Mumsnet learning page link here: www.mumsnet.com/learning/assessment/progress-through-national-curriculum-levels & grid showing expected progress for notional 'average child' by year at bottom of article.
I think you have to take several things on board.
Yes - a reading age of 6.7 when he's only turned 6 is good (and indeed on track) - however others have posted that reading age assessments can turn out high.
1b is slightly below expected level. (but I'm not clear if your DS is in Y1 - but if in Y1 that's not too bad, but the problem is the expected level is regardless of precise age - which is a slightly different kettle of fish and disadvantages summer born children).
The statement that he needed a lot of one on one support may be their impression over the year (but also is a stereotype for summer born children). It may or may not be actuality - but it may be them noting that he needed more direct teacher/ TA support than others (and to be fair that can be very relative).
I had an autumn born DD1 who did far worse than this at the end of Y1 - so my feeling is that your DS is off to a reasonable start and that the relative advantage differences in age provide will decrease from here on out year on year. As you know 4 to 4 1/2 to 5 are huge steps, but 5 to 6 is less of a jumb, and year on year age differences become less of an issue...
I'd take it as your DS is reading well for his precise age, but slightly behind expected performance for all 5 - 6 year olds nationally. You can help by continuing to work on reading and decoding (sounding out) words over the summer. Possibly join a reading club at a local library or see if you can pick up inexpensive second hand children's books at local charity shops/ book fairs/ etc...
There are also lots of free e-books out there for young readers.
Mumsnet has some links here: www.mumsnet.com/learning/ebooks
Oxford Owl also very good - link here: www.oxfordowl.co.uk/
I also recommend the jolly phonics books - which really help practice those sounding out skills and gives good writing practice as well (also lots to colour in - so doesn't seem too much like work - and good to have on car journeys over the summer anyway).
Finally my advice (having had a DD1 who really struggled with reading) is start from where you're at and work on what concerns you, don't worry about the rest.
HTH