Chances are he will be fine, and all your worry will be for nothing! Lol. I can kinda imagine how you must feel, it's the feeling I get when I think about looking for a job and putting DD in daycare...as you know she's very clingy and I'm almost in tears thinking about how she would feel being left for that long without her mum!

Has your SIL spent much time with him? Being in the shopping centre he will have lots of things to distract him, might be a bit harder for you though. Can you put your phone on silent and leave it in your pocket or close to you so you can feel it, that way if your SIL really needs you she can send you a text or give you a ring but hang up and you can meet her outside. I'm sure he'll be fine, but do whatever you need to so that you can feel a bit more comfortable with the situation.