Everything sounds ok health-wise from what you've just written, but obviously I'm no doctor, lol. With regards to the sunken soft spot, if you run your hand over his head, you'll be able to feel the soft bit- if this is sunken in (it's kinda normal for it to sink a little bit due to gravity n stuff when they're upright, but laying down, it should be pretty flat) then it can be a sign of dehydration, as are sunken looking eyes, greyish skin tone, paleness, lack of alertness between feeds, sleeping too much, not hitting developmental milestones, general grumpiness etc... But he doesn't sound like he's got any of those issues, so on that note, it's a plus.
Obviously, some abbies are just naturally small. When you get him weighed, is he gaining every time, even if it's only small amounts, or does he gain some, lose some? As long as he is gaining something (even if it's only 1oz) HV are usually not overly worried but will keep an eye on him just to be sure.
Is he a sicky baby at all? Coz reflux can also hinder good weight gain too.
Google galactagogues (I may have spelled that wrong, so if so, sorry!) and these things can help increase supply. Putting him to your breast & having plenty of skin to skin time can help more so that extra pumping. Do you feed from one boob or both? I stopped swapping boobs in a feed & tried to keep himk just on one boob for as long as possible (till he got really fussy basically, before swapping, but mostly, i never needed to as he was content) so that he gets lots of the richer hindmilk rather than just skimming off the foremilk & just a small amount of the hindmilk from each breast, which can sometimes help too.
If you do go the FF route or end up doing combi-feeding, it's not a major deal. Some BFers are really anti FF, but at the end of the day, if it take some FF to get his weight up and that ends up being what works for you, then go for it; it's not gonna harm your baby & at the end of the day, we all only want what's best for our children!
Do you use a manual or electric pump? I started off using a manual & could only ever get about 2oz (both boobs combined) which was so frustrating & lead me to doubt my supply so much, but I finally invested in an electric pump (medela swing- got it brand new on ebay for £40!!!) & I can get 2-2.5oz now from each boob in about 20 mins, so I can pretty much get a full feed & only pump morning & before I go to bed now.
Really push for the length & head circ measurements, as those 2 measurements combined with the weight will give a much better overall picture... Soemtimes small babies get referred to a peadiatrician or dietician, and if this happens, try not to worry! If it happens, they will do everything they can to make sure he's happy & healthy & see if there are any underlying problems, or confirm that your son is just an ickle one, but is otherwise fine.
Hopefully someone else on here may have some other advice too