I'm surprised your consultant didn't warn you that supplementing with formula will eventually lead to diminished milk supply and eventually a breastmilk shut-down, then early weaning. I'll also bet introducing formula to the mix is also why he's not gaining weight, and your breasts aren't producing much milk. But to answer your question, most childcare books have a formula feeding schedule in them with the approx. amounts of formula they should be getting based on their weight.
If you're really serious about breastfeeding to one year, which is what doctors recommend for immunity and health, to remedy your situation, I strongly urge you to offer your breast more often to the baby, even if he's not looking for it, as this will stimulate your breast to produce more milk.
I also encourage you to supplement with your own breastmilk and get rid of the formula. I know it's easier in a pinch, but health-wise, formula is really hard on baby's system, will never be as complete nutritionally as mother's milk, and has alot of additives.
Start using a medical grade breast pump for about 10 minutes per side or if it's a double-pump, about 10 minutes, when you are not actually breast-feeding your baby. In all, you should be breastfeeding or pumping about every 2-3 hours to prevent a full-on milk shut down. This includes overnight feedings. Store pumped milk in the freezer for future use. You can also pump after baby has had his fill, and freeze and store the extras there also. When you need this milk for supplementing, keep it in a thermal bag, and defrost under running warm water. It's as easy if not easier than formula when you're on the run. At best it keeps much longer at room temp than formula. Even in warm weather.
In the beginning, you will probably see very little milk in the bottles after pumping. Do not get discouraged. Just keep at it, and after about a day or two you will notice a big difference in milk production. Just remember, you MUST feed or pump about every 2-3 hours to jumpstart production, and continue to do so as long as you plan to breastfeed, to keep your supply up. Some women after milk supply is well established (about 4 or more months) can limit or eliminate additional pumpings if the baby is really efficient at keeping the milk supply up on his own. But be sure to monitor weight gain and growth if you do. One up side is, around 6 months you can introduce solids and this will make things alot easier for you as you won't be his primary source of food.
In addition to more pumping time, you should be sure that you are eating, sleeping, and drinking (water) enough to help milk production. If you've been exercising, or tightening up your own feedings/menu to lose baby weight, you could be affecting your baby's milk supply. So eat extra. It's okay and you will not gain weight, because if you're pumping and breastfeeding on a regular basis, you will burn enough calories that you shouldn't see weight gain.
When feeding the baby, make sure you're calm and in peaceful surroundings. Tension and stress can affect supply. Drink water or liquids while feeding, play peaceful music, or at the very least try to go to a quiet place and enjoy the bonding moment and watch your supply increase.
Lastly, get a copy of "The Womanly Art of Breastfeeding" by La Leche League International or visit their website at www.lalecheleague.com for more helpful information on successful breastfeeding. I also recommend purchasing a baby scale to monitor his weight gain. You can buy one on the LLI website or your consultant may have a supplier or one they can recommend. The book TWAB has schedules on approximate weights etc. to have a healthy baby and other important information. YOu can also find a support group in your area via the website.
Support groups are great because often the members (other moms who have gone through consulting/training and sometimes professional consultants) may have the supplies you need such as scales or access to pumps if you need to borrow them. They can also tell you if you're doing things correctly.
If you're baby is really losing alot of weight, this is probably why your consultant is suggesting the formula. She's worried about his health. I say use the formula until your supply comes in and get off of it for the health of the baby.
Good luck and hope this helps.