Webby - I agree with inspiredtowin that you are the best person to make that judgement. A tutor would give you their opinion, but whilst most reputable tutors wouldn't take on a lost cause, they might stretch the truth a little if they want your business. Kip McGrath tutorial centres will give you a free assessment, but as inspired says, it's based on a short session with your child, and of course, they do want your business.
Teachers, in my experience, are reluctant to comment. Partly I guess because they don't want to be held to account if they say a child should pass and then doesn't. These days very few children pass without any preparation and a teacher won't know how much practice a parent is going to do with their child. They will undoubtedly have had experience of bright children failing because they've not done any preparation; and the not so bright children passing because they've been heavily prepared. Also, many primary school teachers don't have children themselves and will be unaware how competitive the system has become.
You need to think about how your son is performing now, particularly in maths and literacy (writing less important for passing the 11+). Have you tried him on any practice papers to see how he gets on with them? Don't be too much in awe of grammar schools. If your son is bright then he should do well there. How well a child does depends, in my opinion, more on their drive and application rather than their intelligence (although obviously they need to be reasonably intelligent to cope with grammar school). I have a very clever but very lazy child and his not quite so clever, but hugely driven younger brother does infinitely better than him at grammar school.
If you are going to go for it, you need to do a reasonable amount of preparation, but in my opinion there's no point going overboard and making a child do ridiculous amounts of practice. There is plenty of advice to be had on this forum about how much/when to practice etc.
You could always put him in for the Kent test as well and consider Wilmington or Dartford (though entry to the latter would be dependent on his score).