Senior Yorkie or Midlife Crisis?

Rambo has been living with me for a while now. He went back to my sister (original owner) for a couple of years and now he's with me permanently.

When I got him, his stomach was covered in brown scabs because of all the fleas. My sister did not do a good job with him... Poor girl was overwhelmed with her new baby and kind of forgot to put his treatment on him?

Anyway, he's all cleared up now with a pink/white belly! Very happy and has a lot more energy! <3 The only problem is, I think the fleas gave him a skin condition... Every now and then he'll start biting his butt right above the tail -- I'm assuming it's itchy there and keeps flaring up. I'm keeping up with his advantix, but it almost appears like after 2.5 weeks, it's ineffective. Anyone else experiencing this? Is it because he's a senior dog now that they're preying on him more? I heard fleas usually attack the weak more aggressively.

Also, I get scared every now and again when he coughs up what sounds like a hairball. Maybe I'm just afraid of him dying of old age, but I know it's silly because Yorkies live way longer than most dogs! He's only 9, after all... : just going to enjoy him while he's here.