Huggies size 6 diapers or even Huggies size 7 diapers should fit a 6 year old boy just fine.
If the 6 year old boy is of average size and not overweight he should easily fit into a Huggies size 6 diaper or Huggies size 7 diaper or other size 6 and size 7 diapers.
Another good diaper for a 6 year old boy is Dollar General Gentle Steps size 7 diapers which are very absorbent and cheaper than Huggies.
Your 6 year old boy may also fit into a Pampers Cruisers size 7, Pampers Baby Dry size 6 or even Luvs size 6.
They also now make a Luvs diaper size 7 which are large enough to fit 6 year olds and even some 8 to 10 year olds.
Another option for your 6 year old or older kid for diapers is Boundless Cuties Youth Diaper in size 8.
The size 8 Boundless diapers by cuties is made to fit kids with special needs and fit up to 85 lbs I believe so a 6 year old and older kid should fit easily into those diapers.
Tape on diapers are much better than pull ups especially for kids who wet the bed or who are incontinent as the diaper that is tape on can be secured snugger to prevent leaks.
Pull ups are okay but they tend to leak and they are just a pull up type diaper.
Pull ups are still diapers but just without the tapes.