Personally I wouldn't mention the separation in the written submission - it was two or three years ago, and your son seems very happy. Mention it very briefly at the hearing - the best technique with a weak point is to underplay it (e.g. "I don't want to make an issue of it").
"He would suit the Grammar system due to his past schooling" sounds as if you think he has an entitlement - a bit risky! (Just my view)
The bullying could be worth a mention, especially if you can get some written evidence from the school to back it up.
For the format I suggest you use bullet points and adapt the Letter of Appeal for Mary (see Q&As, section E12). You may need to add reasons for the particular school - it depends whether Kent follows the Bucks or the Lincs model (see Q&As, section E27).
When I said about suiting Grammar school due to past schooling, what I was trying to do is answer the question of 'Why would the child suit Grammar more than Comprehensive' which I notice the panel might ask (I just didn't put it across very well) - In this case the answer would be that for the last 5 years the school have geared them all towards the Grammar system, they have been set an hour homework every night for the last 3 years, and it's being stepped up until the end of term as every other child in his class have got the Grammar schools of their choice. He has been in this environment for a long time and would flourish if this continued.
Do you think this would be a satisfactory answer? Of coarse I do not feel he has an entitlement, but does the school he has attended so far make a difference in anyway??