We're at the point where most secondary schools in Birmingham and Solihull are academies, or are fast becoming academies. I'd imagine it is a similar situation in Wolverhampton. I'm surprised so many staff are leaving - where will they go?
If the school is a 'forced' academy because of a poor Ofsted/poor results, and has new leadership, it may well be that some staff don't really want to work hard to pull the school/teaching and learning, up. In that case, I guess we can say goodbye to them gladly....
If the school has decided to become an academy because it has a good/outstanding Ofsted and wishes to be independent of the LA/manage its own budget etc, then I can't see why that would be a problem either.
Certainly, a school becoming an academy doesn't seem a reason to leave it, unless there are other concerns, such as issues with leadership, teaching quality, behaviour, etc, that you feel are not improving.
Every child is different, and children have different needs - if your child is in the lower/middle sets in a comprehensive school, you do have to consider whether a Grammar school, which caters for children of significantly higher-than-average academic ability, is the most appropriate place for them.
Tuition aimed at supporting your son in the subjects he finds difficult could be useful (although he is only in Year 7 so I would be reluctant to do this long term) but I am not sure if hiring tuition to prep for in-year entry at a Grammar school, would be the best use of your money/his time.