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,
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....
Put the diapers where he can't reach them and ignore his tantrums. It's time to grow up.