A new child is a drastic change in your home environment. It's a massive change for you personally, and can and will have an impact on your professional life (not a bad thing). It's silly for an employer to think that because maternity leave is over, everything is going to go back 100% to the way it was.
That being said the concerns for a manager or boss fall into two categories. First, what impact is this going to have on the job. For example if your job required you to travel every day and you were rarely home, that may need to change. There may need to be considerations made if you used to work late every day, and now you need to get home on time. Most of these can be handled with a simple high level discussion, one that should have happened before you came back (Probably by someone in HR). Questions like, are child care arrangements made? Do you need to adjust your hours? Are there any big appointments or dates that we need to be aware of? Do you think you can maintain the same level of commitment as before? The goal here is to make sure that as an employer they are not setting you up for failure. However once you answer those questions your back to a full time employee and "things" are no longer their business (more on this latter).
Second, a boss may have friendly concerns. Are you ok? Too much stress to fast? How's home life going? The main point here, is that your boss can (not in this case) try to act like a friend and may be asking technically inappropriate questions, but with true concern. I don't think that this is the case in your current situation.
Once your past the initial "Are you ready to come back?" questions, then your in a weird spot as far as I am concerned. If you answered with "treat me just like before" answers, then you have to hold up that end of the bargain. There are a lot of new, returning mothers that make this mistake. They underestimate the impact that a new child will have on their lives. Most companies recognize this and have some kind of program to help address this. That said, once your back, you should be held to the same metrics as everyone else. If your late, it doesn't matter that it was an appointment for the new child. If you have to leave early, it doesn't matter that it's because child care had an issue. On the other side of the coin, you, as the employee have to hold up your end of "the deal".
Here's the point. While there are some concerns for the company, mostly, these should have been addressed before you came back. Now that your back, rather it's reduced hours or full speed, you should be treated like normal, and your work judged on the merits of your work. The fact that you are a new mother will have some bearing on "things", but not of a nature much more different then any other medical leave.
With some of his off color comments, you could either mention it to HR, or your boss's boss, or simply tell him it's not his concern. Depends on your level of comfort. In fact, depending on your situation and level of comfort, you may start looking for a new job. The workplace seems hostel, but there is also the chance that he just doesn't understand that these questions are making you uncomfortable. Rather you go the new job route or not, it's important that you state clearly, in writing, that your uncomfortable with the types of questions being asked.
You should also consider, based on how long ago the birth was, that your emotions may be more "raw" then they were before you left. That's no excuse for the boss, but before you may have had more of a tolerance for that non-sense, then you do right now. It shouldn't change your course of action any, but it is something to be aware of. Maybe your boss has always been an ass, and now your just more perceptive of it.
In the end you may want to find a more family friendly environment. Rather this is because of "harassment" or not will have to be determined by you. Just make sure that while you are deciding that you document any time that you mention to your boss that your uncomfortable. You should not have to be uncomfortable in your workplace.
