hopnpop Isaac is a wonderful name, really beautiful and manly and also offering the chance for a fun nickname when he is older, if he wants (Zac or Zack). There is no way it is a fashion name.
I hope I am not speaking out of turn but I would agree with bringbackaqualibra, is there anything else could be bothering you?
I would even go further and say are you wanting support from family, needing company from friends or anything at all? I just wondered because a new baby is quite a wrench in life at times, as well as a joy, and it may be that you have some small baby blues that are bothering you. When I had my baby it was winter and I got almost agoraphobic staying in and not wanting to go out! Luckily I had signed up for an NCT POST-natal course which got me out of the house. For ages I got so used to wrapping up warm that I was still wearing a scarf in spring! (Me not the baby).
My mum and auntie both made kind of faces/noises at my daughter's name and now love it.
So to be honest I would not worry what any relatives say or think. They do not realise that in the sensitive state of just having had a baby they can influence things.
Whatever you do, all the best and if you feel very under pressure I would really recommend a chat to the health visitor who may be able to point you in the direction of any support locally.
I really don't mean that in a negative way, I spoke to my health visitor and got a lot of support because I needed it.
Enjoy your little one. He's got a great name and a great mum.