Put the diapers where he can't reach them and ignore his tantrums. It's time to grow up.
i don't know if i am a big help now......but it is a jealousy issues....if you make it a big deal he will win every time.....put him in a diaper and let it be...soon when he goes to daycare he will make that decisions on his own....and then after a month in diapers talk to him again and see if he change his mind....
Perhaps there are some jealousy issues because your youngest 1 year old wears diapers.Maybe you will have to get tougher ignore a tantrum he will throw he is doing that as he knows you will relent and let him wear them.,have a talk to him, tell him no more diapers then let it go, if a big issue is made of it it will continue, its in your hands to set the bounderies.Focus on his positive side , give him plenty of attention, get Dad involved with outside games and Fun,he could be feeling left out if he sees his younger sibling getting more attention,