There is no clear cut answer to this because that would require a definition for the word "income" as used in your order, and only the judge who ordered it knows what s/he meant by it. Here, it sounds as though your order was a product of an agreement, not a contested order, so it sounds like there is no set answser as to what was intended by "income." However, there are strong arguments in support of your position that the profits from the house should not make you loswe your alimony.
First, note that increasing your income to $30,000 does not automatically eliminate your alimony. It only lets your ex apply to the court for a modification which will not automatically be granted. He would have to convince the court that you no longer have a need for alimony because of the increased income, or that you need less alimony, and the amount should be lowered. This would be a weak argument because this is not recurring income, it is one time income in the form of capital gains. So even if it decreased your need one year, it would not necessarily decrease your need in the following years.
But it is unclear that such a capital gain would decrease your need if you used the proceeds to buy a new house with the proceeds. It does not sound like gifting the house to your son in trust would create any problems, and in that case, the sale of the house would result in restricted funds that you could not draw on for personal reasons. Your income would not, therefore, increase, and would not be a basis for modification of the order for alimony. However, you would want to hire a skilled trust/estate/probate attorney to help you create the trust.