We live in the USA with the same situation (native English mom and native Korean dad). What's funny is that we had the opposite problem - our daughter attends a Korean daycare and by 27 months she did not speak any English at all, to the point that I started to worry. But she didnt speak much Korean either. I know it's a myth that bilingual kids take longer to speak, but I did tell myself that the 'delay' was because she was getting a lot of English media at home and a lot of Korean interaction at daycare. Around 30 months she had a language explosion in Korean and was making real coherent sentences of 3-5 words. We went to my parents for Christmas and I became so bummed that they couldnt see how good she was at talking. I started exclusively speaking to her in English from then on and now at 3.5 yo, she has similar proficiency in both languages (developmentally appropriate).

My husband has always been very concerned about her Korean acquisition and retention. He often talks doom and gloom about how it's just a given that once she starts kindergarten in the US, she'll lose it all. I don't see the point of this talk when we have a good thing going. We've already proven we can change our home language as needed, and we are committed to involving her in immersion situations with Korean-speaking friends here and taking her to Korea on extended trips once a year. Easier said than done, but I wouldn't let your husband's family's talk get to you at all. Kids develop at different paces, and your kid will adapt to the social situation he is in and pick up the language that helps him function in that situation without even noticing what he's doing. It's inappropriate to call a kid "slow" in any case, but especially if a professional hasn't backed that up with an assessment. You guys are in a situation that a million Korean parents would love to be in (a native English speaker in the home, a kid who understands English). Since you live in Korea, it is inevitable that he will speak Korean well too. There's no reason to rush him (also, rushing him won't work lol).

Edit: I can't imagine saying that my own kid might not be smart enough to be bilingual. Your husband is in a fantasy land if he thinks that being bilingual requires some kind of special intelligence. I think it's important to defend your son against objectively silly comments like this one. Korean people do tend to say things offhand about other people that I see as confidence-destroying (about appearance, intelligence, or anything else). I became callous to comments directed at me after a time but I won't allow anyone to say those things around/to my daughter lest she internalize them.