I agree with moondog- the number of times I have seen reward systems fail because the child hasn't chosen the reward themselves!
However, I would use a chart for this behaviour.
Correctly, you don't want to reinforce the negative (ie, not climbing), but to gain points for 'having feet on the floor' might work. Behaviours can only ever be replaced rather than stopped, so the goal is to replace climbing behaviour with feet being on the floor- positively reinforce/reward that.
Please don't use minus points, I think it would be counter-productive, and the chart could end up being a stick to beat him with (metaphorically, you understand!).
I would do this at given intervals, as little as a minute apart at first- 'Have you got both feet on the floor? Excellent, one more point!!!/Ah, shame, no point this time!' Perhaps 10 points=one sticker or a prize. If this is too long to wait, make it 1 point=one sticker and buy a LOT of stickers. I appreciate you may feel like a complete looney doing this so often at first, but you can elongate the intervals to 5 mins, 10 mins and so on as he gets the hang of it.
I would also remove temptations where possible, and give him another strategy to gain access to stuff (incidentally, what does he climb to get? What is DS, am I being a bit thick?) he wants. As sooey76 says, label and reward the behaviour you want to see.
Good luck!