Should I ask for an ultrasound?

My 8.5yo son has an appointment with his GP is a couple of days. He has a known heart murmur, and his GP has previously told me that if he still has it at his 10 year check-up, he'd send him for an ultrasound.

The reason for his upcoming appointment is that I just have a niggling little worry that something is amiss with him. He's noticeably pale; he is 52lb and has only gained 2lb over the last two years; he has never been able to keep up with his peers with running or other physical activity and he avoids running as much as he can; he told me, unprompted, that he sometimes gets chest pain and that's why stops running (he does not have asthma).

These are all vague symptoms, I know. I'm not typically a demanding patient, but I'm wondering if it makes sense, in this case, to specifically request an ultrasound of his heart? I don't want to be an unreasonable or demanding patient, so I'm just looking for opinions on this.