i mean no offense, but how can you be so afraid of formula and the "stuff they put in there" and yet give juice and the "stuff they put in THERE?"
before age 1, it's breastmilk or formula. yes, breastmilk is preferable, but there is nothing wrong with formula. it is safe. millions of babies have it everyday.
your child is very close in age to mine. mine is and has always been on the lightweight side. at his 9 month appointment, he was 16 pounds 8 ounces.
what i have done to "beef him up" (he was actually off the charts low at 6 months, and is now back on the charts at 9 months... albiet, in the 4th percentile for weight, and 50th for height):
* baby led weaning, beginning at 6 months. (prior to which he was exclusively breastfed. he is still breastfed on demand as well as offered, many times throughtout the day and night... at least 8 good nursings on both breasts daily). baby led weaning is simply giving your baby good food that you are eating anyway. mine eats whole bananas, sliced strawberries, blueberries, noodles, meat, pizza, you name it. whatever we are having. he can seriously chow down.
we were at a birthday party on sunday and another mom there had a baby very close in age to ours, and she was amazed to see ours handling an ear of corn. meanwhile her baby took a meal of babyfood mixed with formula in a bottle. which there is nothing wrong with that, but baby led weaning is so much easier, and it really does help them gain weight AND feel satisfied.
you just put him in his high chair (with a good bib -- it's quite messy) and put good food in front of him. then WATCH HIM CAREFULLY the whole time and be prepared to intervene in the event that he actually chokes. (note: gagging is different than choking. choking is when there is no sound coming out and the food is genuinely stuck. in which case, turn him over your arm, mouth pointed downward and give a couple firm thumps between the shoulder blade. it works.)
i know some moms around here nurse exclusively to 12 months or more, but i also believe my son would never be satisfied with that. he loves his milk, don't get me wrong. but he also loves to eat. apple slices, crackers, whjole pieces of bread. i can just see that he loves the independence of choosing what to eat from the assortment put in front of him, it's good for his hand-eye coordination, he is learning to try all kinds of new things, broccoli, cauliflower, squash, beans, you name it. he gets chewing. i can see from his poops that he is seriously getting a lot.
oh, and then after he is done (he signals done by reaching up to get out of his high chair), i take him straight to the sink (oh, also strip him down PRIOR to getting in the high chair. nothing but a dipe... makes the clean up much easier). he needs a good rinse off, face, hands, arms, chest, legs, everywhere. and he on his own began grabbing the faucet and putting his mouth on it, he helps himself to water this way, and drinks a pretty good quantity. again, it's baby led because he's doing it himself.
i can't say enough good things about baby led weaning.
and to up your supply of milk, eat tons of oatmeal. also to improve the richness of your millk, eat steaks and other rich, nutritious foods.
good luck with it, you can turn it around for your babe!
Oats are good, fenugreek is good, omega-3s are awesome. Has anything happened recently that might have diminished your milk supply - stress, not eating, pregnancy? After the first few months of super-intensive breastfeeding it's easy to forget that nursing mums need to take special care of themselves. 
Also, just to echo PPs - replace the juice with something nutrient-dense. Even just plain goat's milk would give the kidling more nutrients and less sugar.
Follow Mothering