Colin Kaepernick doesn't seem to be phased by President Trump calling him a "son of a bitch" -- he surfaced in NYC on Monday on his way to the gym ... and he looks ready to play. 

Kaepernick smiled when asked how he felt about the scores of NFL players who took a knee on Sunday following the national anthem protest he started last year -- but the smile faded when he was asked about Trump's newest comments

The ex-49ers quarterback was decked out in gym gear -- wearing a shirt that says "dope" -- and looked like he's put on some serious muscle mass since last season. 

There were questions about his weight and if he's actually training to get back into the league. He definitely looks the part here. 

Kaep was with his girlfriend, Nessa -- one of the biggest radio personalities in the country.