I found his reaction strange exactly for reason you mention that he dated 12 years older guy. Although in the beginning his mum was not supportive of Brian because he was so much older (in scene with Justin and his father she mentions he should not be with Brian because he is already grown up man), even more reason he should have showed more understanding for Jennifer because he knew how it felt when his family rejected his love because of age. His reasoning was that his mom is older so different conditions apply to her? Thats ageism . Probably he was shocked, it was all new to him and needed some time to reconcile with it. Maybe even his breakup with Brian played some role. He was not fully happy so he ventured it on his mum? Because then the proposal comes and he has no longer a problem with it.