HELP! Toddler refusing sleep

Me

Our almost 2.5 year old has stopped sleeping. He moved to his twin bed in May and has been great for months. He’s gone to sleep easily and slept through the night since around 10 months (aside from illness and teething). Suddenly this week, it’s fallen apart. We also have a three month old daughter and need our son to sleep.

He falls asleep at bedtime fine, but then wakes in the middle of the night. For a few nights, he would wake up crying and we’d go in and settle him back to sleep and that would be it. Then he was up twice a night crying. Then yesterday, he just never went back to bed after waking at 3:30 no matter what we tried. He has learned to open doors, and he keeps trying to leave his room. We tried letting him in our bed and he wouldn’t sleep, so I laid in his bed and he still wouldn’t sleep. We tried bribing him with a special breakfast - no go. By 6, we just gave up and everyone was up for the day. He did take his usual afternoon nap, and we wouldn’t let him watch any Elmo since he didn’t stay in bed.

Tonight, he was up at 1, and we put him back in bed six times in two hours. The first time, he was talking about sharks, so my husband explained that sharks are in the ocean and not in his bedroom. We reassured him that he’s safe and his stuffed animals are there to protect him. He kept getting out of bed. We kept putting him back. This last time I opened the door before he succeeded in opening it (our doors are next to each other), and simply said “no, you need to stay in bed” and put him back. He cried immediately. All the door jiggling for hours finally woke up our daughter, so I took her out to the living room to feed her (where I realized I never put the last pumped milk in the fridge and had to dump it - perfect. The whole time our son was crying. After about 5 min, my husband went in and it’s been quiet since.

PLEASE! We all need sleep! How do we remedy this?