Longhaired Missing Patch of Hair Back Thigh and Thinning Down Back2

Help please.

male cat ...mixed long hair Striped. DNA% had showed 35% RagDoll but then I checked a year later and dna was 40% maincoon. 

(He is just a mixed cat IMO)

He is 8.5 years old.

He is my best buddy. 

I'm with and close to him all the time. Day and sleeping at night in bed. 

This isnt just some cat that runs around - i love him. Which is why i'm here.

He is a large, lengthy cat but has always been slender and healthy his entire life

Noticed occasionally  large piles or fur on the floor. Never thought too much into it. I never saw a problem. This happened time to time forever and never showed problems with hair loss. Not at all.

Not an anxious cat. Very attached to me - friendly with full house of strangers etc..

Highly intelligent. Can be needy with me and paw and meow gently for attention. A lot. He has done this since he was a kitten.

I recently noticed LARGE patches of completed missing fur on his thigh.

I noticed this 10 days after I had new carpet installed. I informed the vet of this --- she didn't care at all and convinced the damage had happened over a length of time -- keep in mind this cat is my best buddy - leaning on me now. I work from home. I see him all the time.

anyhow.....My vet said this is 100% NOT behavioral.

Hair thinning went too far up his back for it to be behavioral.

Vet took blood early Tuesday afternoon. Results were in Thursday. I called 2x to get the results info. I didn't get results until FRIDAY around 2pm. 

Thankfully. (Vet says) results were good. All within proper ranges. Endocrin levels were good. (Front desk lady read the voicemail notes to me) 

Now she wants further diagnostics.

I do not notice this giant patch of missing hair? This cat is with me - all the time. 

I had noticed what looked like greasy hair on his back for a while and he was certainly not "fluffy". (He never had been) However, It was enough to make me change food. 

He looked better - never seemed liked greasy because he had less fur. I didnt even think anything of it.

Anyhow.

I feel like i am being Gas Lighted by my Vet. How is Vet SO sure this isn't behavioral, yet I noticed this 10 days after new carpet. 

Yes, i would find some batches of fur on the ground occasionally (a few times a week?) But this happened for his entire life of grooming and I never saw anything like this!

Something just seems off.

Thoughts?