What should my husband wear?

Chino (coloured?) type trousers and a pattern shirt, Floral? if you want to splash out a linen style suit M&S used to do a non crease style.
With lovely grey hair look at some of the pink/ lilac shades of shirt.

It sounds quite a dressy affair from the outfits you and DD are wearing. Perhaps the suit trousers with a smart shirt and fine knit jumper would work for him, with good shoes?

I suggest you let your husband wear what he feels comfortable in.

If he wants to wear a suit fine, as long as he knows he will probably be the only one wear that.

If the thought of being the only man in a suit bothers him, surely he has a pair of jeans or slacks he could wear with a shirt and a tweed jacket or with a jersey?

A kilt....with braces of course.... and full regalia too...?

I've just been looking on Joe Browns, they do some really nice waistcoats too

I like the sound of chinos, smart casual open necked shirt and a linen jacket. Loafers would be nice - maybe slightly more formal than trainers but there are some lovely “smart” trainers for men too.

I second that JaneJudge chinos and a snazzy/jazzy shirt would work well.

Joe Browns do some really snazzy shirts

My DH of a similar build would wear a pair of casual trousers, probably of a chino type, and a long sleeved casual shirt with a small pattern.
Looking in his wardrobe, it would be blue, quite thick cotton with some kind of check pattern.
Take him along to M&S or Next.

My DH would wear a pair of chinos and a casual shirt. He has a linen type jacket which would smarten the outfit up if that's what your DH is more comfortable with. M&S have some nice stuff in at the moment. Casual shoes - Clarks are nice.