Could my dog's symptoms be a sign of food allergies?

my dog just one day out of nowhere,without notice he started to scratch his neck constantly.one day my brother was petting him and found him a really gross and ugly hotspot,puss all dried up.ended up at the vet four times.first time i was told nothing,just a 'give him this and this and done' so im like ok that was easy,i gave him what she said right away,i didnt miss any dosages and gave him all his medicine on time and everything! she even gave him a shot that ugh didnt even work.so then he gets another hotspot,then after his first hotspot he never quit scratching his neck.now everything is itchy,his ears,four paws,back and front legs,all over neck and his chin....lord his chin is the most itching.i give him claritin,recommended by my vet and benadryl too.also recommended by my vet but nothing helps his itchiness!! he even rubs his chin on the carpet or any rough surfaces.we did a food trial,purina proplan sensitive skin and stomach.he has nothing else.its been two months and its not better.it all started when we moved to my moms house,because before that he was GREAT.we moved exactly january the first.we live in the basement...so im beginning to think it might be,the paint?something in the house thats causin this? my moms plants??? or is it still something in the food? please help