Dogs in general, and especially yorkies are pack animals- they have an ingrained need to sleep close to the rest of the pack. If the rest of the pack is on or near your bed, then let him try it out. At 7 months, he should be ready to try, at least. 2.5 lbs shouldn't be too small to be in bed- well, I don't know how big your bed is anyway.
Billi is 2.7 lbs and sleeps in our bed since she was about 6 1/2-7 months. (She's almost 9 months now) and we've had no issues. We do have a ramp for her to get in/out of bed to go potty, or eat/drink. You might try something like that for him? We made a makeshift one at first using two old pillows angled downward for her to slide down.