Feeding in the early hours of the morning is very important for supply so I would try waking for a feeding sometime between 1 and 5 am (ish) as this is when the milk producing hormones are strongest. Mums who don't feed at all at night can run into supply problems. I second the advice to feed every 2 hours in the day, switching sides frequently to boost supply.
Formula milk doesn't contain anything 'extra' that breast milk doesn't have and is no more effective at promoting weight gain so while you need to get more milk in, there is no sense whatsoever in using formula, when you can use breast milk! Also, supplementing with formula very often leads to a drop in your milk supply, so you'll end up with less, not more. You might choose to supplement with formula if you don't feel able to increase your supply or for example, if you need to get Gaviscon into your baby (pretty tricky otherwise without a bottle! Although you could express milk in this situation).
Lots of people think it's desirable to get babies sleeping through the night asap but it really isn't natural and it really isn't good (for BF babies) so don't view waking him as a step backwards or anything like that, your just enabling him to go through the developmental stages he is supposed to pass through, sleeping through will come in time. Sometimes very little babies can get themselves in a bit of a downward spiral, where they don't feed efficiently enough, so they get sleepy and stop asking for feeds even if they need it. How is he in the day? Is he otherwise alert?
This site is really helpful, full of evidence based information and advice:
kellymom.com/health/growth/weight-gain_increase/
kellymom.com/bf/normal/weight-gain/
kellymom.com/bf/got-milk/supply-worries/low-supply/
Of course, determine if there is a weight gain problem first. Some people are small! I had comments from a health visitor and a load of worry at one point but DD is just a small person which isn't all that surprising considering DH and I are both quite little so we were never going to have a big baby.