When a guy wants you to have his child, it signifies he's ready to go on the next major adventure with you. It indicates that he loves you so much that he wishes to have a family with you; he wishes to be a father to your children; and he wishes to see you as a mother because he knows how wonderful you will be as a mother. Don't underestimate the power of this gesture. A man who wants you to have his child sees you only as a partner in love -- no more, no less.
It is important to note that these aren't necessarily "serious" relationships. Having said that, though, I have never heard of a case where someone has actually done them. There may be cases out there but they are rare. The main reason why people don't pursue these relationships is because they don't want to get rejected, especially if the guy is young. If you do decide to take this relationship further, then be prepared for him to move out of his parents' house when you tell him you're pregnant. He will need to prove to you that he can take care of a baby, so allow him this chance.
Also be aware that some guys might try to use this method to avoid having to pay for school or rent. This isn't intended to scare you off, but rather to help you make an informed decision. If you choose to go through with it, then be sure that you discuss with him how he will be responsible for paying for school and rent.
He wants to have a kid with you, which plainly indicates he is investing financially in your future together, he wants you, he wants to care for you and your children, and he certainly wants to grow old with you. It simply indicates that he adores you and wishes to start a family with you.
The reasons why a man wants to have a baby with you can be classified into three categories: practical, financial and emotional. A man who wants to have a baby with you is not doing so out of pity or because it's the easy way out. He knows that having a child will involve lots of work and responsibility, so he needs to know that you are willing and able to take on this task. He also needs to feel confident that you will be there for him and their child when they need you the most.
Practical reasons include wanting to have a family, having room in his house for guests and toys, and saving money on rent or mortgage payments. A man who wants to have a baby with you understands that having a child requires a lot of money, so if he doesn't have enough to give you a comfortable life, then he won't be able to provide you with the things you need to raise a child.
Financial reasons include wanting to establish yourself and your own income, having a plan B in case something happens to his current job, and saving for a house down payment or other investments.