Bradycardia Attributed to Lack of Sleep?

I believe my husband may have a heart condition and I am very worried about him. His symptoms seem to have been misinterpreted or dismissed by his general practitioner, so I am posting this question on my husband's behalf. He is very calm, level-headed, even tempered and quite passive. He is non-confrontational and I have never known him to present a challenge — even when he knows he's correct about something.
I believe he views ALL physicians as authority figures so he did not question his diagnosis.

My husband has always had an unusually slow heart rate, typically around 55 to 60. He started having very brief spells of feeling light-headed a few months ago. The occurrence was rare iInitially, but now the light-headedness has a more chronic flavor and they occur frequently. He says he doesn't know if it's true dizziness so his description is a surreal feeling. He's also had a feeling of shakiness but I haven't seen any tremors, etc. I have seen a huge change in his temperament. He's normally very calm and I don't think he's raised his voice to me 3 times since we met. Now he's short-fused, gets frustrated easily and raises his voice quite frequently. These are all so out of character for him. He's made 3, possibly 4 doctors' appointments in the 13 years we've been together (aside from routine exams). He never tells me when he's coming down with a cold or the flu, so I knew it was causing at least moderate distress when he told me of the lightheadedness.

I noticed a few other things creeping up before he announced the light-headedness. He has always been very relaxed and laid back, but now he can't push himself any further once he tires. He is not lazy by any means, but he doesn't seem to have the drive, motivation or stamina that he used to — and still should at age 34. He's ready to retire long before everyone else, even when we socialize. He typically gets 8-10 hours of sleep per night, sometimes a little more /rarely less. He comes home from work, eats dinner and retires to the couch.
Within 5-10 minutes he's laying down. We'll start a conversation and in mid-sentence I realize he's asleep.

He made an appointment with his GP last week. The doctor examined him and his blood pressure was higher than normal at 135/90 (orthostatics normal) and his pulse was only 48. The doc ordered cholesterol, liver, diabetes, thyroid, electrolytes, CBC — he covered all of the bases for labs. He had his return appointment today and his labs were all normal. His blood pressure today was 115/84 and his pulse was 47. The doctor first questioned him about stress then told him he probably wasn't getting enough sleep! He dismissed him, but did say to return if things worsened for him — he might send him to a cardiologist for a treadmill. I don't know how much worse things can get. My husband didn't agree with the doctor at all, but he also didn't speak up. He didn't tell him just how much sleep he gets each night.

We have a 7-year-old son, and a happy, solid marriage (yes, both of us). My husband rarely drinks alcohol, he does not smoke, has no known health problems and is not taking any medication. His Maternal and
Paternal Grandfathers died from MI, his Father has high blood pressure and his Paternal Uncle recently had bypass surgery. His Mother is diabetic. There have been no changes at home or at work to suggest stress as the cause. His job is stable and it's also the least stressful position he's had. No fever, he denies any general pain, chest pain, pressure or discomfort.

My husband knows I'm posting and we would both appreciate it very much if someone could take the time to give us some input on his health issue. Lack of sleep most definitely is not the problem. It sounds like it could be some type of bradycardia or possibly sleep apnea.
This is sheer speculation on my part as most of the more common disorders have been ruled out.

If you have any thoughts or suggestions, I don't think my husband will approach his physician. He said I get to be "the heavy." Silly man.
It's okay to speak up or even challenge your physician if you're certain he's "barking up the wrong tree." Especially if you feel your life may be in danger. Any suggestions so I don't come across as a domineering wife stepping on her husband's toes?

I thank you in advance.