Firstly, yes, it's fair. In fact, I'd go so far as to say it's mandatory that you at least express some level of concern.
Since you are almost certain he will blow up and stop speaking to you, you have two choices: Steel yourself for the inevitable and go full-throttle, letting it all out with the knowledge that he will hate what you say and react badly to it, but being OK with that; or tread as delicately as possible, trying to not bruise his big dreams, but knowing he could still blow his top.
I would try to approach it (again, with the knowledge that it's probably going to tick him off) not as "You are lazy," but as "Do you realize all the WORK that goes into this? You'll be on-call 24/7; you'll have to keep track of every detail; getting clients will be all on you, as will keeping them; you'll have to do all the _____ tasks related with whatever he actually does; if you screw up there's no one to blame but you; you'll have to file tax paperwork and legal paperwork and keep up with it all the time; you'll have to maintain your office; and so on and so forth" -- wear him out with alllllll the responsibilities, and discussing how he'll approach each one, how he'll still work with the family, etc. If he's got a well-reasoned plan for each thing you bring up, perhaps he's more prepared than you thought; if he doesn't, maybe he will think it through better.
Related anecdote: My father has always wanted to be in business for himself, but while he does decent workmanship, has good sales abilities and is not great with having a boss, these qualities alone do not a good businessowner make. He's had at least four businesses start and fail because he is not good at things like making sure he gets paid, making sure workers get paid, filing taxes, etc. (all that behind the scenes, being the bad-guy-boss stuff -- the tedious parts!). My mother knew he'd be this way and begged him not to do it; he didn't care and did it anyway. Their marriage was already incredibly rocky, but in some ways his business dealings (and massive, massive irresponsibility that had started as things like not doing household chores) were the major, final straw. He ended up massively, massively in debt and working for an even bigger 'the man' than he had been before. Other 'lazy' people can make it work, but his tale makes me super-wary of trusting anyone who shows these signs.