This can be the worst.
You think you are doing everything right, having a great time with their son and then you meet the parents. Mom has that cold stare and dad follows mom's lead.
First thing to know:
It's probably not about you. Parents are protective and they want what's best for their child. If you have only been dating a short while, you have to be willing to put the time in. If you are going to be in a relationship and think he is worth it, the time will pass regardless.
Appreciate the challenge of a reserved family interaction.
If they love their son, it may suck that they are apprehensive to you but know that when you break that barrier, they will probably place you in a high regard because you had to earn their respect. As long as it's never disrespectful or hurtful to you, stay composed.
Always be respectful to the fact that you are the new addition and he's not going to choose his mother over you. Sorry but if you are the ultimatum type, it won't bode well.
Talk. Keep the lines of communication open! If you are struggling with this tell him. Keep talking so you can get it off of your chest and let him know it's important to you. If there is a way to usher in new conversation with the family do that. Let them know you a little better. As we get comfortable with people, it is always easier for the conversation to flow. Be patient.
And as always, be positive. Normally after a few years, the family will see you are a fixture and love you for the support you give to him.