As you are talking about potentially "huge" amounts of tax, you should go to a tax advisor, probably an accountant.
As I understand it, your home will be left out of the CAT equation completely as he has been living in it. Whether you pass it on by sale or by inheritance doesn't matter, it will be exempt from CAT and CGT. That is the law at present, but it could easily be changed in the future. So it might be worth gifting it to him now.
If you leave an investment property worth €300k to your son, he will pay 33% CAT on it. I don't think it matters if you gift it to him now or leave it after you.
If you give him a gift worth €300k now, he will pay CAT at 33% on the thrshold over €225k or around €25k. It will be the same if you leave it to him.
However, if you gift it to him now, you will pay Capital Gains Tax on any gain you have made in the property at 33%. He will be able to set the CGT against his CAT.
An interesting point is would this be considered a property investment for him and therefore would be exempt from CGT if he keeps it for 7 years? I don't know the answer to this, but I don't see why not.
As you can see, you need to consult a specialist.