My 7.5 mo old has been lethargic and throwing up since last night. He will only take a little formula between naps and throws it up. Should I see dr?

Yes. The timing of your visit is also important. If the kid is still wetting diapers every 4-6 hrs you have time to get in with his regular dr. If 8-10 hr since last wet, call the service for guidence or see someone at an urgent care facility.Often your provider can make a basic assessment of babies status by phone and provide you with things to help while you arrange to be seen.
Yes. He need to be evaluated both for the cause and to check for signs of poor hydration.
Yes. Your child may have a stomach virus causing the vomitting. It is important for him to be evaluated for dehydration as this can happen rapidly in infants. So the answer is yes, he needs to be seen by his pediatrician today asap.
ASAP! Definitely.Asap! your baby is dehydrated and needs fluid replacement. Also you want to be sure your baby does not have diabetes especially if there is a fruity smell in the breath or diaper.Diabetic ketoacidosis can be fatal within hours or days.Also need to check for other causes infection etc.See your doctor immediately! or go to the emergency room asap.