Ok, my dd is only 3, but I've had this problem a couple of times now. I'll try to describe how I've done it.
Sit on a chair, and get your ds to stand between your knees, with his back towards one of your legs (eg facing sideways to your right). then, bring your right leg up a little, around his waist and left arm, and put your foot across onto your opposite knee (like blokes sit, sometimes) so that your ds is trapped in a triangle shape between your legs.
Now, put your left arm around his back, and across his shoulders, pinning his right arm under your armpit.
Get him to lean back a little against your left arm, and with your righ hand holding the drops, reach up and put drops in. You use your right hand to pull down the lower lid, while your left hand can reach over from the top of his head and pull up his upper lid.
then reverse it all for the other eye.
It sounds really complicated, and you do need to be quite flexible (!) but I managed to get dd co-operating using this method, as after I'd really pinned her down a couple of times (she's autistic, and REALLY struggled) she realised it was over so quickly if she didn't struggle that she got her chocolate quicker!