Ryan is on his way to Australia after being called into the British & Irish Lions squad.
Jon won his solitary cap to date against Italy in the 2012 Six Nations Championship. He'll meet up with the Scotland squad in Nelspruit where the second match of the quadrangular tournament – against South Africa – will take place next Saturday (15 June).
Jon was in the Scotland tour party last summer to Australasia but a shoulder injury which subsequently required surgery saw him return home prematurely.
Jon told glasgowwarriors.org: "I'm really honoured and privileged to be called up to the Scotland squad and I'm looking forward to it. I've never been to South Africa before, I hear the weather is really good but I'll be going out to train hard and hopefully get into the (match-day) squad.
"I'd been preparing mentally and physically for pre-season training on Monday with Glasgow but now I've had the call-up I'll need to get my head around flying to South Africa tomorrow and potentially playing for Scotland.
"It's the ultimate honour to play for your country and represent them so hopefully that can happen when I go out there."