I recently got engaged on Christmas day. I was just wondering when is the best time to buy the wedding band for a guy?

Probably the same time you go get your wedding band. I've been married for a year and we never got wedding bands. A band would look dumb with my wife's ring because it is custom made and I don't wear jewellery because I have a habit of taking off jewellery and losing it.

Well my fiancé already got my band. He got the whole set!

Is it best to start now?

This should probably be combined with your other thread about when to begin planning for your wedding - https://www.askmehelpdesk.com/weddings/when-best-time-start-planning-wedding-431693.html.

I'd say you begin shopping for a ring when you have the time and the money.

Anytime is fine to buy. Chances are you can have it the same day unless it's a custom order.

We were engaged on Christmas and bought bands the weekend before Valentines day (the following year). There was a big jewelry sale at the mall that weekend. ;)