As a dad myself, I can understand your father-in-law's concern. Also as a dad, I can understand how he views you, though you're rapidly approaching middle age (!), as a "kid". And as a younger generation to him, he feels he has the wisdom and smarts achieved by living through more than you have, and he wants to share that. So, alright.
You only need be practical in your discussion with him. And there need be only one discussion, and he should be the one that starts it.
To show him you are practical and objective about your endeavor, don't use jokes to tell him what you plan on doing if your plans go bust. That would fall as nonsense to his ears because he's having a serious conversation with you, and if you show an irreverent tone, he's going to get the idea you have no plan or are trying your hand at trading as a lark.
Give him the sense that you've done the research and have the ability and understand the risks involved. Give names and details instead of a vague acknowledgement of your studies, and he'll get the impression that you've done your homework. But don't overwhelm him with details.
Also, as others have suggested, tell him seriously about your backup plan, how it is you'll be sending out your resume if in 6 months you don't show signs of being able to make a living. That will show him you're not banking it all on #7, so to speak. Whatever timeframe you give him should be very reasonable for you to accomplish the task, otherwise he'll be back having been proven right. Guarantee him that, for that period of time, you do have sufficient reserves to pay the bills and then some. But don't lie or exaggerate about any of these above matters.
Be sure to thank him courteously for his concern and for bringing the topic up. A mention that a couple of things he said make certainly for thoughts you want to consider (there should be a few) will make him feel he's gotten through.
If that doesn't the settle the matter for a while, you owe me nothing!
Just be sure to make a go of it.