I get you - I think as he said , the more he's pressured (which by the way won't subside) he won't do it - I think perhaps he just wanted a son to make up for the life he didn't get with his own father (issues there?) ..Doesn't see the need (already got the wife, kids and house basically)
Doesn't have his own money to buy the obligatory massive ring
Put off by the massive song and dance of a proposal that would be required
Doesn't fit with the storyline yet?
Waiting for Billie's wedding to be over/ for them to move house etc
Purposely won't do it because she/ everyone is pressuring him
Probably a bit of all the above but number one I imagine he just can't be bothered and doesn't see the need
If Sam doesn't like Surrey and would want to move back,that would be it. Who knows.