advice on how to keep a nappy on

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My 18 month old not only takes his pyjamas off ( has done since about 12 months old), but he has started take his nappy off during the night.
This means every morning for about a week, he has either thrown poo around his cot, or had a wee everywhere!
What can I do, or buy to get him to keep his nappy on?

Would he be able to undo a gro bag?
Could you put him in an all-in-one suit and a pull up?

Swimsuit ( or duct tape ) ir put a sleepsuit on back to front...

DH has taken to 'reinforcing' DS' nappy with red and white tape which says 'fragile' . DS is now getting wise to it and can just about peel it off. DH is considering wrapping the whole way round the nappy now to keep it on...

tights?

at the same sort of age dd3 was a stripped. She didn't really do clothes much and woke me up several times sat naked and pooey on the pillow next to me

we tried a grobag (she undid the zip and got out), pjs and sleepsuits on backwards (she removed them), we didn't try tights, but she could take off all her clothes in the day so I doubt they'd have worked!

we ended up using sellotape wrapped several times around the top of the nappy then cut the whole lot off with scissors the next morning

try putting a gor-bag on backwards so the zip is at the back. He'd need to be houdini to get out of that!

sorry......GRO-bag! No idea what a gor-bag is!

He can get out of the grow bag, without unzipping it somehow? As in the morning, it is still zipped and buttoned up, at the bottom of the cot! So doubt putting it on backwards will work? worth a try though!
Not sure about putting him in tights? My DH would go mad putting his lad in tights - lol. Second thoughts, I might get him some pink ones for a laugh?
I think I will try tape tommorow, and if no joy, I will resort to pink tights!?
Thanks for the ideas, I will try anything instead of having to wash shitty sheets everyday!!!