I would hope that he doesn’t go up and receive communion wearing this sweater.
Words fail me.
However, in that this has never stopped me before

, I submit that the priest needs to take your cardigan guy aside, explain that this article of clothing is not consonant with the reverence due to Holy Mass and receiving communion, and that to wear it to church in the future will result in the ushers denying him entrance to the nave.
The same would apply to “Daisy Dukes”, a man wearing a Speedo, or something so outrageous that nobody — unless they are
really out in left field — would maintain that someone attired thusly is “good to go” for Mass.
I am OK with bringing the dog — even though the dog is naked — as long as it doesn’t bark excessively or otherwise misbehave. If the dog is a service animal, there is no use even to discuss it.