So many wonderful suggestions, thank you all 
ppeatfruit I thought of the Pink Panther movies when we saw the young Peter Sellers in "The Ladykillers". I haven't seen them in years but I'll try and find them on Amazon or Netflix
Singing in the Rain is a great idea Halsall. I've showed him the Paddington Bear version on You Tube
but the real thing would be great!
Laurel and Hardy and Abbott and Costello are great fun. I remember when I was a kid they used to show the half hour L&H's on Saturday mornings. Marx Brothers too Bagel - good call 
You know I always assumed Destry rides again was a western of the old school Gary Cooper, James Stewart sort PrivateParkin 
Certificates can be rather misleading on old movies can't they? DS will say "but it's a PG" but, to me, it's not appropriate (e.g. To Kill a Mockingbird). I guess this is why I've so far stuck to movies I know.
I think I shared the same Saturday afternoons as you two Halsall and PrivateParkin! Bob Hope and Bing Crosby - I'd forgotten about those. How about Jerry Lewis in "Who's minding the store". If found that hilarious as a child. Also "The Million Pound Note"
84CharingCrossRoad - Bringing up Baby is one of my favourite movies. DS will love it I'm sure. 84 Charing Cross Road is a great movie too but not for DS just yet 
We've watched some musicals - Bugsy Malone being one. Also, Little Shop of Horrors, Oliver The King & I and My Fair Lady. I borrowed High Society from my mum last week so that's on the "to watch" pile.
DS has always loved movies PrivateParkin. It's odd, he was a really hyper toddler but stick a movie on and he'd be glued to it. My DP sounds more like your DS! He hates B&W and all musicals (except "The Blues Brothers", which DS certainly isn't watching anytime soon!)
I'm really sorry for the derail. I shall review the TV listings and see what I can find for this weekend. I've found that BBC2 quite often stick a good movie, perhaps not classic era, but something like Chinatown or The Conversation on at about 2am on a Saturday morning.