DD is at Alleyn's.
Alleyn's will be easier for boys than for girls simply because there are many more boys' choices with excellent facilites in this part of London with quite large intakes, e.g. Dulwich, Whitgift, Trinity to name just a few. A lot of boys will also be sitting Latymer and Sutton grammars. For girls, it's just JAGS.
Also, more boys will be sitting 11+ exams coming from states schools rather than private as a lot of private schools go to 13+ for boys.
I also believe that historically it was easier to predict the outcome for boys than for girls - for the latter it's often considered a lottery unless you are at the very top.
If he is at a state school, you speak another language at home, etc - you will need tutoring anyway for ANY academic school, not just Alleyn's. When I say tutoring I mean additional work at home - not necessarily with an external tutor. That could be doing practice papers, closing the curriculum gaps in time for exams, consolidating the basics, etc. You'd be the best judge of that given you know what his school is like. Re English, the only way forward is hard graft and you need to do that anyway for secondary - vocabulary development, literary techniques, ability to write a structured piece in a short period of time, etc. That's not pushing, that setting the foundation for academic work to come. DD had to write TONS of essay in both English and History at Alleyn's in Yr 7 so writing well is a good skill to have anyway.
Finally, I'd like to disperse the myth that these selective schools are full of academic geniuses that you'd have hard time to keep up with if you do tutoring and pass the exam just based on that. In Alleyn's specifically there are lots of kids form junior school (who get automatic entry) and siblings for whom criteria are lower so the actual spread of abilities is quite broad, at least in Yr 7 when there is no setting.
They just did end of year exams and the spread of grades is very wide from low 60s to 90+, and the latter sample is not that large, at least in DD's class. May be 3-5 kids (out 26) who consistently score that - the rest are stronger in one subject and weaker in another or around 60-70% across the board.
So I'd say go for it! 6 month is plenty to get ready with targeted work, but you and your DS do need to put the effort in.