I appreciate the replies, extreme or not.
I will definitely NOT tell my FIL to F* Off. This is a man I respect and I can understand the reasoning behind his skepticism. The last thing I want to do is get into a shouting match with him and possibly damage our relationship.
What I'm going to do is tell him next time we talk that I appreciated his honesty, but in fact he really doesn't know all the facts about trading for him to have said what he did. I will again tell him that I will know in several months whether or not I can truly make a go of this, and that if it turns out I cannot I will reenter the "regular" world.
My FIL is very successful, aggressive, direct (obviously), and a man highly respected and well-known in this community. He's been very generous to my wife and I, but he does have a rather big ego. He's used to giving orders and being in charge and doesn't like being corrected. I also think he considers trading something he could never do, and so it's something a member of his family, i.e. a son-in-law, can't do either.
I too get pissed off when people tell me I can't do something; as a result I am VERY motivated to prove him wrong, and to accomplish this thing that he cannot, but I realize I have to be very careful not to let that mindset cause me to make stupid errors, i.e. increasing my share size prematurely, taking chances I normally wouldn't, etc.
Fortunately my wife supports my trading; at first she was skeptical, but after I explained what I do and she saw how careful and quick to cut my losses I am and how much time I devote to this, she changed her tune. It's her support that matters the most to me.
I love trading too much to give it up for anybody prematurely. I will either succeed, or I will not. I think I will, and that's the opinion that matters the most methinks...