M0nica
He has a cold, not Covid, these are two different illnesses with different symptoms. If it is a cold the worst that can happen is that you and DH get a cold. ~If that would not normally worry you, why should it worry you now?
Just go.
I agree, they have different symptoms. Has he been in a situation where he may have been infected with Covid? Does he live in an area with high or low incidences of the virus.
Are his parents OK?
Your DD did have to tell you , you wouldn’t have been happy if you had turned up unknowing.
I would go, but then I had my DGS when he had a sore throat 2 weeks ago and had the day off school.
When asked by my daughter if I would have him I did ask if she was thinking about the welfare of this old couple. I don’t think it had occurred to her that we may be vulnerable!
You have to do your own risk assessment and then make the decision.
Do let us know what you have decided.