If he's "running around" and nursing good, it sounds like he may just be feeling good, playing hard, and getting tired. Cattle are kinda like us...when they get sick, they usually show the signs: labored breathing, lethargic, depressed appetite. Pneumonia is a concern as well as other bacterial sickness that cause him to run a fever, and if baby calves don't eat or if they scour (diarrhea), they can get dehydrated very quickly. One sign of whether or not he has a fever is to check his nose & see if it's kinda wet like it should be or if it's very dry...that's a sign he might have a fever & needs treatment. Sick calves will usually look like they don't feel good...drooped head, dull eyes, drooped ears, depressed appetite, labored breathing, and not very active. You may see some of those signs and not others, and the more in tune you are with your animals or the more time you spend with them the better you'll be and distinguishing normal behavior from what's abnormal for each one. Keep a close eye on one you suspect might be showing some symptoms...I don't recommend doctoring every animal at the first hint, because just like us, they have an immunity system that needs to decvelop and they should be able to fight off minor illness, but if treatment is necessary, you want to do it before it develops into something that's more difficult & more expensive to treat.
If you're worried about you and you're not sure what to do, try to think of somebody close by who knows cattle who might not mind stopping by and giving you a second opinion. If you're convinced he needs treatment, I keep something like LA-300 and sulfur boluses on hand all the time for treatment of minor sickness. If it were to come down to it, you're better off to treat one that didn't need it than to not treat one that did.