[TOMT][MOVIE][2000s?] Ben Stiller type actor with a hypochondriac step-child who turns out to be his biological son

I vaguely remember a movie from my childhood that I thought maybe starred Ben Stiller as the lead, or another dark haired actor who looked similar to him and was in a lot of 2000s movies. I watched it with my parents when I was REALLY young (probably late 2000s, early 2010s was when I watched it) so I was really confused about how the movie resolved to have the child be biologically his? It involved something to do with swapping something in a bathroom but I was too young to really understand it. Maybe a fertility clinic thing? The main plot of the movie was that this guy was dating this woman he had been in love with forever but who had previously never given him a chance. This woman had a son that he was really concerned about, but the son looked a ton like him and they were both huge hypochondriacs. In the end, the father of the previous boy revealed to the Ben Stiller looking stepdad guy that the son had been Ben Stiller's all along because of the aforementioned confusing switcheroo. I don't think the mom was aware, though. I remember a scene in a restaurant of Ben Stiller critiquing the child's hypochondria (think something along the lines of "no you don't have tuberculosis because .... those symptoms actually sound like cancer!" or something like that. I feel like it might not have actually been Ben Stiller acting in it, though, because when I google Ben Stiller alongside the synopsis the only movie that comes up is The Heartbreak Kid, which doesn't sound like the plot of the movie I remember at all and which doesn't seem to contain any actual children amongst the characters (at least based on the synopsis) whereas I think the child was a major plot point in the film I watched?