Although this isn't much of an answer, I think that you will be fine. You seem courteous, and you are thinking ahead which puts you ahead of most. Maybe you will get lucky and have a sympathetic seat mate who would watch your son for a couple of minutes.

I am a FA and I have never mined holding a baby for a few minutes while the mom goes to the lav. Just pick one that doesn't look busy - or one that isn't eating!

I don't know about Delta's policy, but on my carrier we are not allowed to hold babies while standing. I think it is for unexpected turbulence and liability. Kinda makes sense!

Good luck, and enjoy your flight!