My nieces 5 month old will NOT stop crying...?

When she goes to work I keep the baby for her but I have my own children and I have to run my household so I cannot constantly hold him. After making sure he's changed, fed and burped, I have to help my kids with projects, clean, and cook and I can't do all these things while cradling him on my hip. I have purchased a vibrating bouncer, a swing, a sit in baby gym, a colorful baby floor mat, a pacifier and several other things but NOTHING helps except when I pick him up. He instantly starts cooing and smiling when he's being held but as soon as i put him back down he screams at the top of his lungs. My niece NEVER puts him down when she's home. She constantly walks around with him in her arms and now I can't stop him from crying. Any suggestions please?