Stamp duty is always on the market value, not the actual sale price where this is less than the market price, so here stamp duty will be on €450,000. There is a relief called consanguinity relief which halves the stamp duty between close blood relatives so this would apply here.
If you have not given any gifts, and your son hasnt received an inheritance before from a parent then he will not have to pay gift tax on a gift of €200,000 though it will be taken into account in the future for any further gifts/inheritances ( the threshold is currently nearly half a million).
Finally you may be liable to CGT, depending on the purchase price and taking into account any works done on the property and the sale and gift costs and outlay and your own small gains exemption as a gift is a disposal for the purposes of CGT.