Should i be worried?

With every day that passes, the healing progress will be that further along so after 6 days, the incision should be getting to the point it is pretty much healed even if the collar should still be on, or at least you should be watching!! If you are concerned, get your vet to take a look and advise.

Shame you had him castrated so young - it's too late now, but for future reference, it's better to leave this until the dog is at least a year to give him a chance to mature a bit, and you to decide whether castration is totally necessary. Unless he had retained testicles or other medical need and even then, the surgery to 'find' the missing testicle(s) can be done later than 5 months.

As long as the skin is closed and there are no signs of infection then he's ok. When wounds heal they itch, so its normal for him to want o lick it.
If the area turns bright red, is hot to the touch, or there are red lines radiating out from it then he would need to go back to the vat asap.

You're suppose to be constantly supervising him and not allow him to lick it.

Perhaps contact your vet for advice, phone calls are free, as to what is recommended to do when you can't supervise, perhaps a different collar and/or size.

Keep an eye on the incision. As long as he doesn't tear out the stitches it should be ok.

Poor puppy