Just because someone is old, doesn't mean they're a nice person. Sometimes, if they're lonely, that's the harvest what they're reaping for their choices.
Yeah, I'd really like to hear what his son says about him, for example.
It sounds like he's a bad combination of leech and exaggerator. Any sort of friendly contact is an excuse to latch on to someone, and start sucking. No matter how much time or energy or effort you give to him, he'll want more. You'll visit him at the hospital, drive him to class, be a partner in his business, and he'll turn around and expect you to nurse him through his illness. And you can't believe even his most basic statements, because he's always exaggerating to make himself look more important, his family more neglectful, his illness more dire, his business ideas cooler....
It does occur to me that you're all excusing a *lot* of bad behaviour on the grounds that he's old and lonely. If a 40 year old man with whom you had a casual acquaintanceship with was phoning you multiple times a day, insisting that you drive with him to class (or meet him to take the bus together), trying to coerce you into being business partners, and displaying a borderline obsession about your pregnancy, you'd be thinking more "restraining order" that "it's sad that he's lonely".
This behaviour sounds so ingrained in him that unless it's a recent development due to senility or other mental illness, he's probably been like this his entire life.
The business part you can address directly. Email him, and explain that while you were happy to chat about his
business with him, you've realized that he has way over-estimatd how involved you will become. You will not be becoming a partner or employee or in any other way officially involved with his
business plans, although you wish him the best. If it comes up in conversation with classmates, make it clear that you aren't involved with the business, regardless of what he says.
For the clinic, I'd talk to the nurses and your gynecologist. Mention that you've recently become acquainted with John through a class, and he's displaying more interest in your pregnancy than you're comfortable with. You know that they have strict rules about information/visitors, you just want to make it clear that John is not authorized for *any* information or visitation when it comes to you. Then they have the information they need to keep your privacy.
For the rest - back off hard. If he calls and you pick up, you can make excuses. It's too late, it's too early, you're busy, you're expecting a call from a client, you're in the middle of dinner, you're about to go out the door, etc.
Become a brick wall when he tries to push boundaries. He wants to drive. "No, that's not possible". He's insisting on you drinking his beer "I told you I don't want to drink it. Drop the subject *now*" and then ignore him completely, no matter what he says.
I suspect, however, that you won't be able to manage to maintain a casual friendship with someone who has this level of leechiness, and eventually you will drop the contact completely.