My two have both been bottle fed from birth. expressed BM and formula for a couple of weeks and then formula. With DD I found that with BM she was hungry at more or less exactly 2 hours but the same amount of formula would last her 3 hours. A 3 hour feed routine is fairly common even with very little formula fed babies as formula is harder to digest than breast milk, that said although I tried to keep to 3 hours if they were hungry earlier they were fed earlier.
How much is he taking at a feed and how long is it taking to feed him? At 3 weeks he should probably be taking around 4oz a feed and should be going at least 2 hours really if he is taking a proper feed. Which flow teat are you using?
Keeping awake is very hard when they are so little and this will improve as he gets older! I used to strip mine off a bit to wake them up, play with their feet, stroke their palms etc and a nappy change usually did the trick for a few mins at least
What was his birth weight and what is your HV calling excess weight
. Has he jumped up the growth curve much? DD was 8lb 11oz born and gained around 0.5lb a week. At 1 year she is about 24lbs. In fact both of mine jumped up a few centiles in their early weeks and then followed a curve for a while. My friends breast fed baby has been gaining about lb per week in comparison!
HTH a bit, sorry for the questions but hopefully we can help a bit more!
Laura