This is not the time to tell you son that he is being unreasonable if you want a good relationship with him and his family in the future!
What you can do, is consider whether you would have been overjoyed to make a 225 mile journey with an infant. Are you frankly not being unreasonable to have expected young parents of a first child to do so even once?
The important thing is to consider your husband's well-being. Is he up to this journey, and what help will he be in a household with a new-born baby and a toddler?
Could he manage on his own at home, if you go?
If you read some of all the many other grandparents' threads about feeling hurt at being excluded when a new baby is born, you will realise that for some ,to us incomprehensible reason, midwives, and other advisors are recommending parents being alone with the new baby in order "to bond"
You may, like me, consider this frankly ridiculous, but it is what those your son and daughter-in-law regard as authorities regarding birth and its immediate aftermath are telling them. So don't rock the boat.
You could consider saying, "Son, we would love to come and help, but your Dad is just not up to it, so soon after his hip replacement, and you know how helpless he is when I go away."