Miss Kitty had this problem last year...I eventually took her to have her
shaved. Found out she had tapeworms..due to fleas that got into all that
fur. Even with a flea product, she was still prone to them. You may need to
get him trimmed down by a professional.

Common problem.
Don't know about shampoo, it might help to loosen them a bit so you
can more easily brush and comb them out.
I usually just cut them out if I can't get a comb through them.

Big Mama usually gets them in the summer time when she's outside more.
I think the mats are caused by her inadvertantly rubbing against
plants or trees
with sticky stuff on them.

The dandruff stuff could be an unrelated problem.
In cold climates, sometimes it's just as simple as dry skin.
So a certain shampoo might help with that also.

Trixie is a domestic British (moggy) long hair and she is continually
getting them even though she gets a good brush every two or three days.  The
other week, I found lots under her arm pits despite thinking that they
weren't there 2 or 3 days previous.

I just get her all relaxed and then snip them off with a pair of sharp
scissors.  She is usually quite obliging and has got used to the attention.
I make sure when doing it that my fingers are between her and the scissors.
Sometimes the knots are so tight that they get close to the skin.  When this
happens, I use the end of the scissors to tease at the knot until I've got
it off the skin, and then snip little bits off until it comes off
completely.  Too dangerous to take those off in one go.

The dandruff may simply be down to dry skin, or the cat may be suffering
from eczema or other allergic skin condition.  A vet would be able to tell
and may take a scrape of skin just to make sure there isn't any sign of
infestation.