Suggest Treatment For ADHD

I have a 4 year old who will turn 5 in May. He has issues with comprehension. My thing is he doesn t seem to understand some task or questions. He also has delayed speech and working with a speech therapist and is improving. My wife and I are really worried. We took him to the neurologist and she said he has ADHA and that he needs speech therapy. His little brother who is 2 soon to be 3 next month has loads of vocabulary than his big brother. How can you help us help our son? Thanks