This is a well-known bug.
And unfortunately the only solution to that is to load a previous saved game, otherwise it won't work.

When you first meet him (he will give a nice 500XP bonus if you get into Khorinis by swiming) and talk to him, when you ask about Lee, he'll tell you what you need to know and that he can take you there. This is when first option of him escorting you appears.

Secondly, when you talk to Vatras about missing people and everything, when you leave, Vatras will give you the ornament fragment so you can take it to Saturas. When you talk to Lares about this, the second escprting option appears.
I strongly recommend you you going to Water Mages first, because Lares is immortal during this time, and you get full XP by simply following him (I mean he kills everything). But when you go with him - be sure you take level up a bit, because there are some goblin skeletons in those ruins, and if you can't kill them, you'll ose the XP (Saturas kills them if you run to him, but you get nothing from it).


On a side note: keep in mind, that at some point, you'll go with Lares again in the middle of the forest behind Sekob farm. During this time, Lares can kill all wargs, shadowbeasts and lizards and whatever else you want.
How? Easy.
When you run behind him and you stop, he stops too and waits for you. You just walk off-road a bit and lure those monsters to him, and he does all the job for you. Just don't go too far, or he will disappear.