Overreacting?

Question
My boyfriend has two cats.  One is a 2 year old male (purebred) british short hair (Cuddles) that he has been cat sitting for his brother for about 5 months.  The other (Grendle) was a stray he took in when it was quite young.  We estimate her age to be about 6-8 months.  The cats at first did not get along.  Grendle was playful and Cuddles was used to being the sole cat.  Now they get along fine.  Grendle has not yet been fixed because my boyfriend is broke.  Cuddles has been fixed.  They are both indoor only cats.  
When we first got Grendle, she had fleas and worms.  We had her dewormed and got the back of the neck flea stuff (we made sure to wait until were could be certain she was old enough).  Then Cuddles got worms.  So we bought over the counter dewormer that you add to the food.  So far, the worms haven't returned.  After a flea bath and collars, the fleas are gone too, but both cats are still scratching.
I'm only giving that background because I have no clue what might be relevant.
The problem I'm seeing now, is that Cuddles' hair is thinning.  At first I convinced my boyfriend that he was thinning out for summer, but then I started to think it may be something else.  He's always had a thicker coat.  Pretty much just the back 1/2 of him is affected.  I've never really seen the pink of his tummy, but now it's showing through.  Should we be worried?

Answer
This is not quite my area of expertise, but I can suggest that you see a vet.  It could be allergies or a skin disease called feline endocrine alopecia.  I strongly suggest you contact a veterinarian, despite your fonancial situation.

Another thing (unrelated to the hair loss) that I am wondering is has Grendle been tested for Feline AIDS and Leukemia?  Strays sometimes acquire these diseases which are transmissible to other cats.  Good luck.