Going it alone, I think, would be fine. I had PND with DS but I figure that the dr will simply medicate me during my pregnancy to stop that happening again.
That doesn't guarantee you won't have PND this time, sadly.
It's obviously a gamble either way, isn't it? But make sure you don't let the bloodiness and excitement cloud your thinking so you're not considering whether you will be able to cope and your son will be able to cope if things go pear shaped. Even the ideal outcome of DP being happy will still be a huge adjustment for your son and there's no saying how it'll affect him now or in the future.
But if DP turns out to be feckless, abusive, irresponsible, shit with money, a cheat, or simply loses interest (you're still in the honeymoon period) and leaves you before or after the birth, would you feel able to cope with possible PND, a confused six year old who has dealt with a lot of upheaval, a brand new baby, financially practically and emotionally?
That's not an unlikely scenario. If you go into this do it with your eyes open and think very carefully not just about how you'd feel and cope but about your son too. It saddens me to see PP say 'don't worry about your son he'll be okay' as if that's a foregone conclusion and he will just have to adjust.
Sorry to be the voice of gloom btw. I hope it works out for you. Just wanted to add some of my perspective to the other posts saying 'if you want the baby then have it, everything works out fine in the end'Z