There are a few points you make in your post that leave me with more questions than answers, but I presume they are things you would have given consideration to yourself.
How long ago did your other son marry, and get financial help from you? How much did you give him? How much difference would there be between the gift you gave one son to the amount being requested/demanded by the other son for an 'investment property? Does he resent his brother for having had financial help? Could this potential resentment be long-term?
Why is he helping with your money? Does the other son also help with your finances? Does he 'help' or does he have any sort of control, like Power of Attorney etc? Could he just 'take' the money if he chose to do so? (This would certainly worry me)
Can you afford to give him money? Loan him money? Invest? If this is money set aside for your retirement you might need to say no and explain IF there is any left after you die he and his brother can do as they wish with their inheritance. Is it just the 2 brothers or are there other siblings. or others that might inherit?
Why do you feel you can't refuse him? Is he emotionally blackmailing you, or threatening, intimidating etc?
If the son you gave a gift to is better with money, or needed it more that would be understandable. Is the son trying to get you to fund him having an investment property good with his own money? Do his own finance give you concern? Has he ever betrayed your trust or used your money inappropriately before? Do you trust him? Would he be prepared to give you a large share/percentage of any investment property rent/sale?
Usually investments or investment properties are something people who have money or save money for have. If your son clearly doesn't have his own money why should he have yours to invest? If you wanted to buy a property or invest the money you have I'm guessing you would have done so. With the limited information you have given I'd lean more towards saying no to him. You may need this money for yourself. If he hasn't saved his own money to invest I'm not sure I'd trust him managing my finances - are you sure he 'helps' you and not himself with yours? If you have any doubts you need to ensure he is not the sole control of your money. If you do decide to help him maybe draw up proper legal paperwork protecting your assets, making it clear what you expect, what is a gift compared to a loan or in lieu of inheritance. Maybe you need to specify that if you need the money for care or just living expenses at a later date he has to give you what you need, even if that meant selling the investment or giving you all the income from it. It doesn't sound like this is his intention - it sounds like he wants to use your money to give himself, and only himself, an income. Does he work? Have an income? Have the ability to save so that in the future he can fund his own investments? If he hasn't been good with making decisions like saving his own money, how can you trust him to make a sound investment? Would I be wrong to suspect he is looking for an easy way to make a living, to get an income, using someone else's money - YOURS?
Just to play devil's advocate here...what if he invested badly. What if he squandered the money, gambled it, went on a spending spree instead? What if he did buy a property and later found it needed more money spent on it than it would be worth? What if he got an investment property as a renovation project then mis-managed the renovation, or spent more than the ceiling price for properties in the area? There are so many risks involved that, for the most part, people risk THEIR OWN money and not that of other investors...only very rich people have others manage their investments, usually a portfolio or something.
If you cannot afford to potentially lose this money, if you have doubts and/or concerns (and your post suggests you do) then the answer has to be NO. Calmly explain to him that you need the money and he will have to wait to see if there will be any inheritance money. In the meantime he needs to pay his own way in life, save for any investments he wishes to make etc. You need to look out for yourself first. If you have a huge lottery win or something you could reconsider. Please don't be bullied, cajoled, intimidated or threatened into parting with your savings. Good luck