Unfortunately I don't have specific advice on when to go in, but I can say that going in at certain times and not at other times when he's just calling can work out. I know it's all about being consistent, but here's what I mean. This is going to sound tough, but dd has always needed to be alone to sleep. After we put her down for a nap, she used to call out for us (just like when she wakes), but we VERY rarely would go in because we knew she was just stalling and didn't actually need us. This did not teach her that we're not coming at other times however. She still knew to call when she woke up and that we'd come get her. Now she calls when she wakes in the morning and after nap when she's ready to get up. She even wakes sometimes mid-nap on strange days, but won't call for us cause she knows she's not done sleeping....not sure how she learned that one as we didn't ignore her or anything mid-nap...she just fusses a little bit and goes back to sleep. I know that doesn't help you with timing, but thought it might be helpful in knowing that after you figure out when to go in, you can go in at certain times and not other times for just calling out and he'll learn the difference. Now I wouldn't do that for actually crying like you're talking about of course.
