We've topped up some of Espen's feeds as he was losing weight and no longer had the strength to suck properly to bring down my milk. I'd only do it under the advice and monitoring of a lactation consultant/MCHN as if you do it it takes away the opportunity for your baby to stimulate your supply for more milk. If he is feeding constantly it could be the same problem I had as my little boy was trying to get more milk all the time... the problem was the constant sucking was wearing him out and making things even worse. A great tip I got from the LC was to feed then put him in the sling and get him off to a good sleep so he had the strength to feed well again when he woke up (rather than feed/nap/feed/nap for hours as I was doing).
We have pretty much only given him one bottle in the evenings- we did it after a feed when he just couldn't settle and appeared hungry. That way the time between me feeding him wasn't being too messed up so that my supply could still be boosted.