If your 6 year old son wants to wear diapers again you should let him as diapers are the most comfortable underwear and especially when the diaper gets wet and swells up or he has poop in it.
That is good that your 6 year old son was wearing his brothers Huggies diapers as it shows that he loves wearing his diapers and you said he was wetting his pants so that is probably why he wants to wear them.
He's hoping you will put him back in diapers and you really should.
Yes you should put your 6 year old son back in diapers for wetting his pants.
If he is wetting his pants he needs back in diapers and many 6 year old boys wet their pants and need diapers full time.
You could put your 6 year old son in Gentle Steps size 7 diapers or Pampers Baby Dry size 7 diapers, Pampers Cruisers size 7 diapers etc.
Your 6 year old would be very happy wearing his diapers again.
I would never potty train my kids and leave them in diapers.