M.B.
Until 1 year, their main source of nutrition should be from breastmilk or formula. Solids are just for practice eating and so they can experience the tastes. They should never replace breastmilk or formula (i.e. if your baby starts taking less from the bottle/breast, you should cut back on solids).
My baby was very petite as well. My ped was also very relaxed; she fell over 70 percentile points before the ped finally suggested we try increasing her intake. Realize that, as long as he's growing (especially if he's staying in the 5th percentile -- staying on his growth curve) and hitting milestones, he is probably just supposed to be a skinny baby.
Don't give more solids. In fact, I'd cut back. Solids are less caloric than breastmilk or formula and fill them up more so they drink less -- sort of exactly the opposite of what you want to do. Also, you should only give solids after they've taken a full bottle (whatever is the normal amount) or a full nursing session; that helps to make sure they are getting enough calories and nutrients from the milk before adding in "practice" solids.
Also, breastmilk is generally more calorie-dense than formula and is digested faster (they'll be hungry again sooner), so I would try to nurse as much as possible, especially on weekends. When my baby's weight was getting too low, I started dreamfeeding before I went to bed (picking her up asleep and nursing her) and nursing twice instead of once in the night. It helped, and I've actually come to love that extra cuddling! Now we're back down to one middle of the night nursing.
As far as how much formula goes, if you multiply their weight by 2-2.5, you get the number of oz of formula they should take. If he's 12 lbs, he should take 24-30 oz of formula a day. However, you're also nursing, so cut that back a little. A BF baby will take about 25 oz a day (breastmilk becomes more caloric as they grow -- they don't increase the amount like formula fed babies do).