Hi, I really think you need to get in touch with one of the b/feeding helplines - NCT have one, as do La Leche League - I'll have a look for numbers for you in a second.
I've had 2 very sleepy feeders who lost 10%+ (ds1=15%, ds2=12%), and I did the EBM topping up. But I had a different outcome to the feeding with the boys, largely due to dd, who came in between them, lost only 8% or so, quickly put it back on, and gave me the confidence that I COULD feed a baby.
Ds1 - rubbish support in hospital, was told all sorts of rubbish. Day 8 - lost 15% of birthweight, sleepy etc. Tried topping up with EBM, still lost weight, tried one formula top up, still lost weight. What I should have done is fed him every 3 hours (as you are doing) from the start, but no-one told me that - they told me 6-8 hourly. I ended up b/feeding for 13 months, but always with formula topups - 3 topups a day. Which made me sad.
Dd - born at hom,exclusively b/fed till 6 months, carried on b/feeding till 13/14 months. No problems.
Ds2 - born at home, excellent support from mws, and MN! Day 4 - lost 12% weight. Was already feeding every 3-4 hours, started giving EBM - after every feed (he was very sleepy, and the idea is it is some 'easy' calories for them - they don't need to work to get them - this can then perk them up to encourage better feeding) - still lost a bit of weight, had about 2 formula topups of 1oz each, started putting weight back on, I gained confidence, ditched the formula, exclusively b/fed to 6 months, just weaning him now at 15 months.
So do have confidence in yourself. If he needs feeding more, then lots of skin-skin, try feeding him in the bath, wake him up if he drops off in a feed, try 'switch feeding' (swapping from side to side, as the increased spurt of milk wakes them up a bit). But do get some support, preferably from someone who can observe a feed. Keep an eye on nappies - they should be wet, and poo should be yellow - a few poos a day I think. Feed every 2-3 hours in the day, and don't let him go longer than 4or possibly 5 hours at night. Wake him up if need be!
Good luck - you're doing really well, and giving him such a good start.