Will's dad could have come back
I don't get why they act like it was impossible for them to still take the trip later on. Not like he could help some unexpected business coming up.
shareWill's dad wasn't too busy with his business and needed to delay/cancel their trip together. That was just his latest "I'll be right back" excuse, before leaving with no intention of ever coming back. He didn't even want to tell Will himself, but tried to have Phil and Viv tell Will for him, but when they refused to do his dirty work for him, he tried to duck out and leave the problem to solve itself.
Then Will appeared before he managed to disappear in time, resulting in their final goodbye/disownment for each other.