Spoiler.......................................................................................................................................................................................................................................................................................................................................
ML was attracted to MC but he didn't feel it's right to be attracted to someone 11 years younger than him. At first he's just trying to be a benefactor. He kept trying to keep the right distance with MC, but after the failed kiss (ch17) he started distancing himself even further. Then MC got himself an alpha boyfriend. it started off fake but MC gradually tried to go with the flow since his heat was near and maybe he could try to move on. Later ML and his friend coincidentally met them on a date and he noticed MC was in his pre-heat state and was pheromone-marked by the boyfriend. It definitely didn't please him lol. Everyone assumed MC would spend his heat with the boyfriend (well they almost did but MC ended up backing out (ch20)). Later in his rut ML and MC accidentally had sex (next week's ch22 probably). ML felt really guilty about it bcs he knew the reason he couldn't control himself like usual is because of his attraction towards MC, so he completely cut contact with MC even made him stop working at the friend's house but still let MC continue with the tutoring. Later when MC found him and told him he's pregnant, his first thought was the boyfriend got him pregnant when he was in heat, so he acted a bit mean towards MC bcs of jealousy. But after he saw MC was really distressed he dropped the mean act. After that he'd be completely supportive towards MC even while he still didn't believe the child was his.
Tldr: ML is generally gentle, he acted cold to distance himself from MC and he's only mean at that specific moment because jealousy got the best of him.

How does ml take a 180 in the future when he’s like this now??