I had a big boy too and he did not start sleeping longer stretches until he was 2 1/2-3 months old. We were SO sleep deprived. I was desperate!
He also loved to sleep anywhere BUT his crib or bassinet. We had him in his bouncy seat for a long time. Eventually I moved it into his crib. Then I put him to sleep in the crib. When he'd wake up and be fussy, I'd put him back into the bouncy in the crib. Gradually he slept in his crib longer and longer without being in the bouncy.
We tried everything. That secure sleeper mentioned below he HATED. He cluster fed a little, but we could not make him eat when he did not want to. We had an awesome bedtime routine down, which has helped later on but not when he was so young. He did not take to the family bed at all unless he was on one of us. It's so strange how all babies are so different.
I wish I had tried this earlier, but we found the techniques in Dr. Harvey Karp's, "Happiest Baby on the Block" very helpful. I learned how to correctly swaddle him and it immediately helped him sleep in his crib better. Also, playing waves loudly all night was a life saver. It may be worth a try for you, just in case it is the right appraoch for your young man. Our son has been a great sleeper, with a few short blips once in a while. He's 2 1/2 and just recently we had a few months that were really tough. I remembered back to the tough early days, and kept telling myself that it would pass like it did then.
I also wondered if a crib vibrator would help? He loved motion and vibration. I see them all over the place now...Target. When I was looking they were no where to be found.
You have a lot on your hands with 3 kids under 5. One 2 1/2 year old does me in! I hope sleep finds you soon.