'Riverdale' Actor Ryan Grantham Afraid He Could Be Brutalized in Prison

The "Riverdale" actor convicted for murdering his mom could end up behind bars with the worst of the worst -- something he and his attorney find gravely concerning, considering he's a puny guy.

Ryan Grantham -- who was sentenced to life in prison in Canada -- is at risk of being brutalized, or worse, if he's forced to serve his time in a maximum security facility ... according to his lawyer, Chris Johnson.

Johnson tells TMZ that Grantham is small in stature and slight in build -- standing a mere 5'2" and weighing about 100 lbs. While the guy is 24, we're told he passes for a teenager ... and looks exactly like the frail characters he's played in movies and on TV.

Johnson is concerned his client could be "physically, psychologically and sexually intimidated" if he's locked up in a max security prison. Basically, he feels Grantham doesn't belong there, not just because of his physicality ... but because, in his mind, the setting for the punishment doesn't fit this specific crime under these particular circumstances.

Johnson tells us his age, size and mental state during this saga should be considered by prison officials -- suggesting even if the crime fits the bill for someone to be thrown in a maximum security prison ... anyone with eyes can see he simply wouldn't fare well.

We're even told the judge who presided over the murder case actually recognized the potential concerns if Grantham ended up in a max security prison, and even recommended a medium security facility.

Thing is, Johnson says the judge has no final say about where he's sent ... it's up to correctional authorities.