As-Salamu ‘Alaikum sister,
May Allah (swt) reward you reaching out to try and find a way to correct your situation for His sake and for your family.
The first thing you should really do in this case is to seek forgiveness for being engaged in a haram relationship. Living together and having relations together outside of marriage is indeed a big sin in Islam. I think you realize this, and it is very important that you sincerely repent for this. This might be quite difficult to hear given that you have been living this life for such a long time and now you are expecting a child together.
Regarding delaying marriage until he gets financially stable, whilst it is ideal, it is not a must for a marriage to occur or even to be successful. The fact that you have been living as if you were married and now have a child on the way should make this even less important. You have a child on the way now. Whether you are married or not, this will inevitably have an impact on finances and, therefore, should not be what prevents you from moving forward in marriage. Your parents should understand this. Naturally, as this child’s grandparents, they will likely be willing to do all they can to support you.
Marriage is very important in Islam for so many reasons. It can be a source of great happiness, a source of support, the means to increase the population, and a way to have your emotional and physical needs met in a way that is pleasing to Allah (swt). Marriage will protect you and prevent you from falling into sin. There is much evidence in the Quran and Sunnah to support this.
“And of His signs is that He created for you from yourselves mates that you may find tranquillity in them; and He placed between you affection and mercy. Indeed in that are signs for a people who give thought.” (Qur’an, 30:21)
“And Allah has made for you from yourselves mates and has made for you from your mates sons and grandchildren and has provided for you from the good things. Then in falsehood do they believe and in the favor of Allah they disbelieve?” (Qur’an, 16:72)
“Narrated `Abdullah: We were with the Prophet (SAW) while we were young and had no wealth whatever. So Allah’s Messenger (SAW) said, “O young people! Whoever among you can marry, should marry, because it helps him lower his gaze and guard his modesty (i.e. his private parts from committing illegal sexual intercourse etc.), and whoever is not able to marry, should fast, as fasting diminishes his sexual power.” (Sahih al-Bukhari 5066)
It was narrated from Anas that there was a group of the Companions of the Prophet, one of whom said: “I will not marry women.” Another said: “I will not eat meat.” Another said: “I will not sleep on a bed.” Another said: “I will fast and not break my fast.” News of that reached the Messenger of Allah and he praised Allah then said: “What is the matter with people who say such and such? But I pray and I sleep, I fast and I break my fast, and I marry women. Whoever turns away from my Sunnah is not of me.”” (Sunan an-Nasa’i 3217)
To name but a few. In sha’ Allah, if your fiancé realizes these things and the importance of marriage and fears Allah (swt), as well as the haram involved in continuing as you are, then he will do all he can to ensure that your relationship is made halal as soon as possible.
A marriage ceremony does not have to be anything big, and if you have been living as if you were married already, then nothing will really change. In sha’ Allah, if and when his father is ready to help out in the future, then he still can even if you are married.
Most importantly, I would urge you both to seek repentance. May Allah (swt) forgive and bring you happiness and contentment in your marriage and grant you a righteous child.