Our Siamese is losing hair on his tail and is spreading to his body..

My 6 year old Siamese male has recently been losing hair from the tip of his
tail and it is slowly moving in a stripe formation to the base of his body.
He seems to be eating and using the litter box regularly as well as playing
with our other cat (1 year old).

I've been reading all night about possible reasons but I have fallen short
of any reasonable answer.

Thank you for your help.