Hi J.,
First, I have to congratulate you for losing 105 lbs. Wow! I lost 35 a few years ago and even that was a mammoth task, so good for you!
Now, I will assume you are breastfeeding. If you're not, then disregard this note. My son is now almost 17 months old, but was similar. He never slept through the night and when he woke up I would feed him back to sleep. Unfortunately (and I realize this is not encouraging news) he did not sleep through the night until I stopped breastfeeding last month and then like magic, within a couple of days he started sleeping straight through. I think once he wasn't getting the milky cuddle, waking up was just less appealing.
All this said, I would never recommend stopping breastfeeding before you're both ready. What I can recommend is something I read, unfortunately only about 2 weeks before I decided to wean my son.
The issue may be that he is waking up genuinely hungry. Don't let people tell you that after a certain age they don't need night time feeding, because if he is eating every 3 hours throughout the night, he will be getting a good number of his daily calories during those feeds, making him less hungry during the day and therefore not eating the amount of solids he needs, which could ultimately sustain him through the night. You say he's underweight and this might be part of the reason. After 6 months, babies need solid food to give them the calories to bulk up and it sounds like your little one is getting far too many of his calories from milk.
The advice I was given was to make the nighttime feeds as short as possible. Take him off the breast as soon as he's finished. If he will take a bottle, maybe offer him some water or well diluted juice at one of the night feeds. I found that my son would take a bottle better from my husband.
The other thing is to fill him full of as many solid calories as possible during the day. That means: no juice or milk, only water and only a small amount at the end of the meal so that he doesn't fill up on it. The exception to this is if you can get him to take some well diluted juice at his afternoon feed that would be great. That way he will be hungrier and eat more solids at dinner and hopefully that will sustain him better through the night. Also, and I'm sure you do this, but make sure that the food he is eating is hearty: meat, pulses, potatoes, fruit and veg. Absolutely no empty calories: cookies, etc
I got all of this information from Gina Ford's 'Contented Little Sleep Guide' if you want to read it from the horses mouth.
Take care and good luck, D.