He will be too young for beavers if he has just started school, buyt as others have said a lot oaf packs do have a waiting list. However when DS started I was told it would be lucky ifhe got in before 7 and we were offered a place on his 6th birthday.
DD was so tired after school when she started school we endedn having to srop he dance classes, so it might be better to find something on a saturday morning.
You will probably find out gradually by word of mouth or reading school/community centre/village hall notice boards or by internet googling.
Get his name down for your local Beavers pack waiting list and by the time he is old enough to start he should have enough after-school energy to cope with it. He may not be too tired yet but by the end of the Christmas term ,with nativity play rehearsals/performances/Christmas party etc he will be very tired if my ds and his reception class mates were anything to go by.
Swimming is a very useful skill and getting confidence in the water is an important start to actual swimming itself. I'd look out for a weekend swimming class for him. Football/Rugby/Tennis/Cricket/Dance/Gymnastics/Trampolining etc training/clubs are best kept to a weekend too at this stage in his life as he really may not be up to it after school as this term progresses. Maybe once a week after school at the most.