Don't worry, love! You haven't messed it all up! It was his first night in a BBB. That typically stinks! LOL! Plus, y ou have a 5 month old. That will often screw up older sib sleeping b/c of sibling rivalry etc. The sibling rivalry often doesn't happen initially b/c big brother is enamored of the new person in his house plus is sort of hoping that baby will leave eventually! LOL! Around 3 months old is when the older sib typically starts to act up.
So! What to do now:
*We don't do PU/PD with children over age 1. Instead we use a technique called WI/WO (walk in/walk out). With WI/WO you go in to soothe child when they full on cry (not just fuss or yell). You soothe without taking them out of bed. You can sit on the bed and hug if you want. Then when child is calm, you leave. Child may begin crying immediately, as soon as you stand up. That's OK. Continue to exit the room, and count to 5 before coming back. Then soothe again. The first day or two you try this, it may take 2 hours or more! But that's to be expected. After that you will see immense improvement. I realize you have an infant in the house. But you're just going to have to steel yourself to having TWO kids up for a couple of nights in order to nip this problem in the bud. Otherwise you're going to get caught up in some really annoying AP techniques that will be worse than a couple of sleepless nights! LOL! If you want, start this on a weekend and enlist the help of friends or family to come over during the day so that you can take a nap and recharge. I also encourage parents to wear an ipod while doing this with a nice book on tape so that you aren't as frustrated with what's happenign with the child.
Once child is calm and you leave, do NOT go back in unless they full out cry. Fussing and yelling don't count.
*I would discourage you from staying until he falls asleep. He's been an independent sleeper and will be again. Staying until he falls asleep just encourages this behavior. Esp. when you've got a baby in the house.
*I would encourage you to have DH help. Tell him how WI/WO works, and then leave him to it! PLEASE stress to him that leaving the child to cry it out is NOT preferable. That can lead to detrimental side effects like having a child fear his room etc. In other words, it might work a night or two, or even a week, but it will eventually backfire and believe me it is really tough to sleep train a child once they've been through CIO, especially an older child.
The advantage of having DH do WI/WO with toddler is that it eliminates the competition for YOU that he has with sib. Since you probably feed baby the most (or all the time if BF!), toddler is going to want to fight for your attention. If DH does the sleep training, then DS1 doesn't have to try to compete for your attention at all. A lot of moms have had great success with DH doing WI/WO b/c LO doesn't try to pull on daddy's heartstrings as much, LOL!
*No parents sleeping in child's room! Again, that will lead to a really bad habit that will be hard to break.
Hope that makes sense! Let me know how it goes or if you have quesitons.