Caring is one thing, 90% of the South American mothers are like that, mine included, so I understand Daval, especially if he's from South America too... but going in person to take care of your adult son is a totally different thing! and I know that Daniel lives with them and all, but he's someone who is show as mature and independent, Alex knows that and he's right when he said "he will be fine" because any capable adult can take care of themselves unless they are seriously ill, if that were the case I think even Alex would cancel his festival plan to help his son, but well... every character is dumber than Daniel, they never suspect anything, they never ask for details, it's just another shitty excuse that will work because the plot requires it.
Now I wonder what shitty excuse they will give him when he calls in the next days, if Lana says that Daniel is still sick, Alex will think it's something serious and return, if Lana says that Daniel is fine, Alex will ask her to return because the festival didn't even start yet, the camp will now be like a whole week long, how do you explain that Daniel has been sick a whole week? then they need to return before Alex does... again, the sick excuse works better if it's just for one night or the son is like 12 or so, someone who can't take care of himself, that's why I said that Daval writes with his "other stories" in mind, he doesn't know how to write an adult son.
I feel this whole camp setup would have been better in a more honest approach you know? unfortunately the irrelevant trip to the uncle's house ruined everything, but this whole thing would have been better if:
-Daniel tells Lana about the camp
-They pretend to have a serious beef for X reason (like Daniel not going home that night)
-Lana talks to Alex and proposes a mom/son camp to try to fix things with Daniel and since Alex is stupid he would believe that.
All this would make more sense because in the first issue the story makes clear Daniel and Lana have a bad relationship, so Alex would think they are in bad terms like in the past... no big lies, more freedom, but Daval prefers that fake drama that leads to nothing because there are no consequences at all.