I work in elderly social care and the one thing I have realised is absolutey nothing is guaranteed. People can have children and still end up facing old age alone. Having siblings and children isn't a guarantee that they won't be "alone". In my wide experience, its those that have little friendships and community that are lonely, having no siblings doesn't make any difference for thus.
I mean this kindly but unless your son is going to through the next 70 years not speaking to anyone then he won't be alone. I think it's quite sad that we seem to place so much emphasis on siblings and the nuclear being the only form of support and meaningful relationships a person can have. Good friendships are an essential part of life and can provide as much meaningful support as family and sometimes even more.
I have two siblings and I'm not in contact with them, I've had a parent diagnosed with a serious illness and there's been radio silence from them. They don't bother with my child, don't offer to see them or buy presents or get in contact with them. Am I alone? Of course I'm not, I have friends a husband, in laws and colleagues who have all offered much support than my colleagues.
We aren't designed to be isolated in nuclear families, we're meant to have support from friends and community. I think life must be more lonely for those than only rely on their immediate family for support and companionship than make an effort to make deep meaningful relationships with friends and community. You can have siblings that move away or don't get on and if you've gone through life to believe or been brought up that family can only provide meaningful support and not invested in friendship, then this is a lonely place to be.
Life will be very different for your son growing up. I'd take comfort that families are getting smaller and there are much more one child families now. I see this is a positive in stopping people from looking inwards towards their families and actually branching out towards others in making meaningful relationships and connections.