We Just Started Giving My 8 Year Old Son A

We Just started giving my 8 year old son a whole 1MG tablet if Guanfacine for his ADHD instead of half in the morning and half at night and he has already had 2 nose bleeds that I know of should I be concerned? I know 1 in about 1,000 people who take it get nose bleeds but what is the cause and should I look into getting him back in with his doctor to change his medication?