It could be a number of things:
1) Is it really messier than it was in K, or does it seem to be messier when compared to his grade-level peers? As MoN mentioned, if this is the case, it's possible he's dysgraphic - the messy printing (reversed letters, not following lines, difficulty with spacing etc) looks much more "normal" in K-1 than in later grades, when nt kids' handwriting legibility and neatness keeps improving at a much faster rate than dysgraphic kids.
2) Has he had his vision checked lately? My dd who has poor eyesight has really sloppy handwriting - when she can't see. When she's wearing her glasses and when she's not seeing double vision etc, she has very neat handwriting. OTOH, I can absolutely tell just by looking at her papers when she's written something and wasn't wearing her glasses.
3) It's quite possible it's simply a factor of changes in *why* your ds is writing - in K, chances are the writing samples you saw were handwriting-specific exercises, and the attention was on proper formation of letters, spacing etc. Now that he's in 2nd, chances are he's writing to *write* - sentences, stories etc - so now the focus is on content not handwriting. If that's the case, you should be able to sit him down with a handwriting worksheet and watch him write and all should look a-ok. I wouldn't necessarily worry about this either - most kids' handwriting *while* writing for content does improve with time.
4) Check his pencil grip - does it look normal? Is his posture ok? If not, try having him correct his grip and/or posture and practice holding it all correctly when he writes.
Best wishes,
polarbear
