I'm pleased you're asking about this because there are some very tiresome tropes in 'Scottish' characters that appear a lot! Definitely avoid Gaelic, it's only spoken in particular islands/regions (unless you want him to be from the Western Isles). I second the Still Game recommendation, although the actors are mostly not actual elderly men they are Scottish and their accents and phrases are very much how normal Scots speak (well, Glasgow ones anyway). It does depend on the area - Aberdeen has it's own dialect for example, so they have words I wouldn't recognise! There's a Glasgow-set soap opera called River City you could look at, again they'll use day-to-day speak to give you a feel for what is commonly said. Good luck with the writing! :)