Red/black rash underneath my dog. On Tummy and inside back legs

Hello vets. Please could someone give advice on what this could be. He is up to date with all flea and worm treatment but they did change the brand could this be a reaction or skin allergy? 

Hello - this could be skin inflammation, possibly with a yeast infection.  It may be caused by damage to the surface from scratching eg fleas or mites (I hear that you're controlling for fleas), hormonal changes (endocrine), the effects of allergy (common), or diseases such as skin cancer (rare).  Frustratingly, there is likely nothing I can recommend that you can rub on this to make it go away forever.  People tend to think of a rash as a one-off disease, but skin disease tends to be a chronic problem;  it repeatedly affects the body's most copious organ and can need careful, multifaceted management.  I reccommend repeated visits to the vet, and questions like 'what is this treatment and what are you expecting to happen going forward?' and 'What are the long term prospects?'  I hope that something there proves to be useful.