I have never given this any real serious merit, the idea of my child being "embarrassed" about a safety issue. When my son had to wear his bike helmet while his other friends didn't, he got over it. Now that he is 18, and not yet licensed, he is only able to get rides home from 2 kids we know well. If they are not available to give him a ride, he has to call home and I will pick him up, while his other friends just hop in any old car. My rules- for safety. Is he embarrassed? Maybe, but he copes with it and just says, "That's my mom- the safety freak."
When my daughter is Kindergarten age, and still harnessed, I will tell her she is safer, and will ride that way until I decide otherwise. Young children do not necessarily become embarrassed unless some adult says something like, "Oh, you still ride in a baby seat?" Kids usually side more with their parent's values until about age 10-12, when peer pressure kicks in more. As our kids grow up, hopefully more and more of their peers will be harnessed longer, and it will be the norm. HBB are NOT appropriate for kids that do not meet the 5 step test and are not mature enough to stay properly restrained. 5 years old may be to soon, it may not be, it depends on the child. Your husband's fears are not uncommon for men- they don't want their sons to be ridiculed, or "babied". Safety comes first in my family.