ADHD medication is only effective for about 80% of individuals with ADHD. Your son could be in that 20%. OR, the ADHD diagnosis may be incorrect. If he is still concerned, you can encourage him to seek out a new evaluation and 2nd opinion.
Feeling foggy and tired all the time can also be anxiety, or any number of physical issues (like thyroid or adrenal). So a physical from his GP might be helpful as well.
ADDitude Community Moderator, Parenting ADHD Coach, Podcaster & Author, Mom to teen w/ ADHD, LDs, and autism