I am concerned that if i accept the secondary school the panel will think I am ok with that
Rest assured that they won't - they know very well that everyone is advised to accept the offered school while awaiting an appeal.
or if I put the private school they may think well they can afford to go private so we don't need to seriously consider him at appeal!
This shouldn't influence them either.
I would suggest that you accept the offered school for the time being to keep all your options open. It's what an appeal panel is likely to view as the sensible thing to do. It doesn't stop you changing your mind and going private at any point in time.
Also it asks if I would like to put my name down on the waiting list for any schools. Can I put my 2 catchment grammar schools in here just in case I am successful at review?
You wouldn't be allowed to go on the waiting list unless and until successful in the first two parts of an appeal (i.e. the panel decides that the review was not 'fair, consistent & objective', and also decides that your son should be deemed qualified).