Hey! Take my advice with a grain of salt because each baby is unique. My daughter is now 5 months old but she too had the same sleep pattern as your son. Several of my friends and sisters recommended that I cut out her late night/early morning feeding cold turkey. They did it with their babies and it worked for them in 3-5 days. For example after the 4-5 hour stretch when your son wakes up for another feeding at 2 or 3 am, rebundle him, rock him and put him back to sleep but don't feed him. He will most likely cry for a while but each night he will cry less and less until he no longer wakes up for that feeding. I am too sensitive and couldn't do that. However, I was struggling with PPD and was desperate for sleep, so my mom intervened. What a saint! Rather than having me breast feed my daughter at 2am my mom would give her a bottle but would slowly reduce the amount of milk each night by half an ounce. Starting out with 3oz the first night, 2 and 1/2 oz the next night, etc. slowly going down to a half ounce. When my daughter would still wake up, my mom would rebundle her and put her back to bed with a pacifier but not feed her. Within 9 days my daughter was sleeping through the night (7-8 hours straight). Now at 5 and 1/2 months she's sleeping 10 hours at night. It's great!
Regarding the startle reflex, my daughter too struggled with this. We tried the Swaddleme Blanket by Kiddopotamus but it didn't work well. She would still wiggle out of it and wake herself up. However,the Miracle Blanket worked for us like a charm! It wraps around the baby like a burrito and keeps him/her snug! We got it at Buy Buy Baby and loved it! I seriously recommend it. Around 4 and 1/2 months she grew out of it and rather than buying the next larger size, we quit bundling her cold turkey BUT when we did that we introduced a sleep buddy to her (kinda like a baby blanket with a bunny head on it). I put some of my breast milk/scent on it and gave it to her. She loved it instantly and slept/sleeps with it unbundled. She no longer wakes herself up.
Again, I recommend either cutting out the 2 or 3am feeding cold turkey or by weaning him slowly with a bottle. Also, I recommend the Miracle Blanket at Buy Buy Baby. It's worth the $30!!!!