His Glasgow teammate Jackson, who was first choice for Scotland during the World Cup has not recovered from a hamstring injury he picked up in RaboDirect PRO12 defeat to the Scarlets at the start of January.
The 20-year-old Weir will battle it out with Phil Godman, Greig Laidlaw and Dan Parks for the fly-half role but Robinson admitted he'd been impressed with the youngster's ability to step up since joining the squad.
"Duncan has trained with the squad for the first time and has settled in well," said Robinson.
"He is competing with the other stand-offs for a place and has had a good season with Glasgow Warriors.
"Ruaridh is working on his hamstring and started running and got involved in training on Tuesday. But he has not played consistently, or even back-to-back matches for six weeks, so we have to manage him so that he will be available later on in the tournament."
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