Is it forbidden to dream frequently about someone?

If love enters the heart without any effort on your part or transgression of Allah's boundaries, there is no sin in it, provided you fear your Lord and adhere to His limits, and strive to repel it. If marriage to him is possible, and he is a man of religion and good character, then that is best, for the Messenger of Allah, peace be upon him, said: "Nothing like marriage is seen for those who love one another." There is no harm in you proposing marriage to him directly or through an intermediary.

However, if this love arose from unlawful looking or leniency, then you are sinful for it, and repentance is obligatory upon you.

The dreams you see do not hold you accountable, as they result from what you find within yourself regarding this person.

If marriage to him becomes easy, then so be it. Otherwise, strive to turn your heart away from thinking about him.