I read one thread about how to pick up Theo's boulders but that was from years ago & I don't know board ettiquette about reviving really old threads- and nothing there helped anyway. So how do I get up boulders which are too heavy for me? How do I enlist my companions help when there is no dialogue option for it?
Try putting it in you inventory. then drop it on the ground. One of you followers will pick it up then.
But if they are too heavy, can you even put them in the inventory in the first place? Can you drag them by using the ALT+click method? I probably used Strength of Earth when I did that quest, and if you don't have the spell, perhaps you can buy or steal a few scrolls and see if they stack with each other? Might not be worth the quest reward, but it's a quest in a CRPG - you have to complete it! :-o
Try to pick one up, if it doesn't work, drop an item, so you'll create an improvised container. Then you can see the size of the item.
If two items are in the same tile a loot pile is created. That loot pile is an improvised container, which lets you see how much space the boulder occupies. It didn't occur to me, but it's true that if you just try to pick a boulder up directly from the ground, it might not fit into the inventory. Though I didn't read that the OP got any messages about there not being enough space for that item, or whatever the game says when there's not enough space. In fact, he specifically asked what to do if the boulder is too heavy to lift.
I think if you drag them, you ruin his crops and botch the quest. Even if a follower picks them up he won't open the wagon and put them in for you. I think the correct way is to have the strength high enough so your max carrying capacity lets you hold them. Use a spell/scroll, or an elixir, or an item that increases strength (axe ST+1?). Now if you can just barely hold it, but can't walk, then you can have a follower carry it for you then transfer to you and you to the wagon.
That should work. aslong if you have enough space in your inventory. I believe you can pick up items and drag them in your inventory even if you'r heavy encumberd (and not being able to walk anymore).