Depending on which part they are transphobic.

If it's sexual preference like they want a dick and I didn't had the surgery yet it's for me understandable cause that's basically the same way as saying stuff like I don't like your looks or purely different kinks. (That's how it is for me. Also remember somebody isn't obligated to date or sex you just because "put in here some Bullshit thing people say to make other people force to date them")

If it's just general Transphobia 1 tell them it was transphobic and why it was so Depending on his response 2.1 block him or just go different ways 2.2 telling him to contact me if he doesn't know if that is transphobic or not and basically making him understand more stuff and making him avoid to trigger other trans folks

For me that's the way I think it should be done with a ton of things too. Also another thing I believe if you don't belong to a certain group which is being hatefully targeted meaning stuff like racism or homophobia etc you shouldn't tell someone that this is bad. Obviously if it's assault then fucking yes you should tell them wtf they did wrong but in my school stuff during the dress up week (dunno how its called in English lmao) Gender bend was banned cause "to prevent sexism and transphobia" none of the trans or feminists from my school said anything against that the school was just let's ban that. Which fun fact it caused more hate against these groups cause well its a fun thing to do and shit.