All of my siblings have different last names. It never bothered me. We are all at least 6 years apart so we didn't really grow up together. I'm closer in age to my cousins And we all had the same last name. My grandma had 8 kids,6 boys 2 girls. All the boys had girls and we were all a couple years apart lived close to each other and went to the same schooLs.
I do agree that 1st and last name is a bit much to ask. On the 1st name i would propose to make his dad's 1st name your son's 2nd name. It ll be honoring his grandfather, but still give him his own identity, as i feel having the same name in 1 family can be confusing.
On the last name, i'd let that depend on your relationship with your boyfriend: how long are you together, how solid is the relationship, perhaps tell him to marry you (if you want that) if he wants the child to have his name.
To me, giving the child his father's last name is a recognision that he is the father. I wouldn't worry about the different last names, it is quite common these days (in my own family too). But you need to feel secure enough with your boyfriend that he is truly going to be the father (and i mean not just biologically) to the child, and will be there for you. To me being married helps to feel that security, but more so that my husband and i have been together so long (14 years!) I don't know how his relationship is with his other son? I'd look at that too in your situation.
I can see wanting the last name, but also the first name? That's a lot to ask. I've always suggested that unless a couple is married, the child gets the woman's last name. So many women have written in here to say they wish their child had their own last name because their boyfriend left and now they have a different last name than their own kid. It gets hard to explain to the elementary school, when putting the child on your insurance, etc. I know that this reason might not apply to you, of course. :)
My mom gave all 3 of us our dad's last name so my 2 sisters shared a name and mine was different, but it never really bugged me I just understood it was cuz it was my dad's name