I agree that they didn't have the father-son relationship in the Gunslinger that they had later, but I think there was a love and respect that grew between them. And then of course he dropped Jake. He might have done it quickly without much thought, but he was guilt stricken afterwards.

When you take into account that guilt, the love between them, and that awful paradoxical mind splitting they both experienced (which is one of the major reasons for Roland's angst, rather than just pining for the Lost Boy- he was gradually going crazy) the emotional impact when they reunited is understandable.