To go out for a drink with an old friend

[BigBumandMumTum]

He's male.
I used to work for him and we were friends who had a laugh.
He's old enough to be my dads older brother (maybe not my grandad lol)
He moved away 15 years ago and has just come back.
He's married, I'm married and he's asked me if I want to meet for a catch up (we are Facebook friends)

My husband thinks this is entirely unreasonable

Not unreasonable at all unless there's a huge backstory here?

Is your husband generally weird about you having male friends?

Yes he is.

No backstory at all, I knew him well before I met husband

Not unreasonable at all

Perfectly fine.

I'm going to a gig next month with a male friend because I know DP wouldn't like that kind of music. He's taken female friends to his work Christmas parties before now if I haven't been able to make it.

Couples should trust each other.

Not unreasonable. Your husband is! Must be insecure.