I gave pretty succinct rebuttals to this in the comment section for The Young Turks' video on this subject:
Just search for comments made by "A86", my YouTube user name.
There is no evidence to support the notion that the kids and grandkids of Hispanics will have lower IQ since IQ studies have never studied more than 2 generations (parents and their children or adopted children).
IQ in and of itself is a doubtful measure of intelligence. IQ doesn't test or measure common sense, wisdom, creativity, improvisation skills, social skills/social intelligence, life skills, work ethic, knowledge, how well you adjust to new situations/experiences, etc. It simply measures the working capacity and efficiency of the human brain to perform certain mental tasks in a certain amount of time. The higher the capacity to retain rote information or the higher the efficiency at recognizing certain patterns the higher your score. That's an iffy measure of "intelligence", which is likely a multifaceted thing. Not some simple and fuzzy "g".
Add in the fact that, due to the Flynn Effect, the IQ disparity gap has decreased over time between many groups.
You can read more about the Race and IQ thing here:
I think the Wikipedia article does a fairly good job on the subject. So do scientists like James Flynn, William Dickens, Wicherts, etc. UHM also gave a good explanation.