Surprise background on the army of 40,000:
This army's nation had been allies of the kingdom for many years, dating back to the king's father's father. Their nation however, had fallen upon poor times, their agricultural capacities having been severely limited by a drought. Growing food had become impossible and the ground water was drying up, rendering wells unproductive.
In trying to feed their people, this other nation had attempted, after years of suffering, to strike a deal with the nation who's army was smaller (army faced by the king's men, not the son's). They were promised food and water clean water.
One week after the larger nation's aide had begun arriving, the larger nation presented the recovering nation an ultimatum: "Help us overthrow the kingdom, or we will besiege your nation until every last one of you have starved to death."
Seeing little option, this smaller nation begrudgingly accepted. The larger nation was to send 4,000 men to ensure that every man, woman and child took up arms, and fought to the best of their ability (the larger nation, seeing a nation of 36,000 farmers who were only just recovering from starvation, felt that 4,000 would be more than enough to keep the weak and primitively-armed smaller nation in line).
Already weak from lack of nourishment and marching through treacherous winter conditions, with 80% of this newly amassed "army" suffering from pneumonia and frostbite, the men of the nation approached the 4000 soldier contingent of this "army", asking to allow the women and children to return to the village. The response: a swift and heartless "no".
Upon seeing this 40,000 person army amassed just miles from the kingdom's walls, the king's son zealously and indiscriminately slaughtered all 40,000 men, women and children among their ranks, not recognizing them as the humble, hard working member of their neighboring nation. A nation that once helped feed the kingdom.
The basics (if you're trying to further cement the population's hatred of the son): grant the son the glory of having killed them all with his light cavalry against insane odds. But then announce, in front of all the people and the king's soldiers:
"Though you may have slaughtered your opponents on the field of battle in the face of 10 to 1 odds, you've failed to realize one thing. That is, 9 of the 10 foes your men slaughtered... those were our neighbors, our friends, our allies, our fathers' allies. And worse, the women and children among their ranks. You and your men are certainly brave and powerful warriors.
Tomorrow, YOU and your cavalry will lead us into battle." (Just doesn't seem like medieval days were the days where you wanted to first into battle).