Recently, my two-year-old son has been taking off his diaper at nap time. It's usually full and dirty and makes a big mess.

Care to read what you have written? If it is terribly full and dirty, why exactly do you want him to keep on wearing it? It might be utterly uncomfortable/wet, that might be the reason that he is trying to take it off!

The advice is to not let him continue wearing a diaper which is so full and dirty. Before he starts becoming uncomfortable, simply change the diaper.

Observe what happens when he wears a clean and fresh diaper. Does he still try to pull it off? If yes, then duct-tape may be the solution. If not, then have mercy on him and change the diaper.

Making him unable to take his dirty diaper off by tape or by making him wear the diaper is cruelty, IMO.

Recently, my two-year-old son has been taking off his diaper at nap time. It's usually full and dirty and makes a big mess.

If the diaper is terribly full and dirty why exactly do you want him to keep on wearing it? It might be utterly uncomfortable/wet, that might be the reason that he is trying to take it off!

My advice is to not let him continue wearing a diaper which is so full and dirty. Before he starts becoming uncomfortable, simply change the diaper.

Observe what happens when he wears a clean and fresh diaper. Does he still try to pull it off? If yes, then duct-tape may be the solution. If not, then have mercy on him and change the diaper.

Making him unable to take his dirty diaper off by tape is cruelty IMO.