That's a tough thing to deal with yourself Liro 
Saying that, I would try my hardest not to project your emotions onto your DC. While you may see it as a tragic, terrible thing, your DC need not think like that, and it doesn't need to be anybody's fault.
I would, as the PP suggests, have a general discussion about how families come in all different shapes and sizes with different dynamics. If you are concerned about him feeling lonely, take comfort in the fact that he doesn't have to feel this way and it can be addressed by sensitive parenting. Take him to see friends and invite school friends round once he has established good friends.
I would also consider looking into professional advice/guidebooks or even googling if you are really worried. Many kids will ask about siblings out of curiosity/because their friends have them too, so it's not always because they feel deeply sad they don't have one.
I'll be interested to read any other responses too; we have 1 DD and are planning another, but we have put off doing it this year and have decided to wait for a long time, until DD is around your sons age.It's also worth talking more to your DH if you think not having another will be truly detrimental to your own emotional wellbeing. It is an important thing to agree on. Good luck 