'Dia merasakan itu' mean you, he and another person know what he feel, ex, he feels hungry (dia merasakan lapar) mean you, he, and another person know that ''he" is hungry
'Dia merasa itu' this case is another person didn't know what "he" feel I make sure, another person ask to you 'what is he feel?' (merasa apa?/dia merasa apa?)
"Merasakan or merasa" is acceptable but this case suitable use "merasakan"
That my opinion ^_^
This case is difficult to explain, just use your heart