A bit of context: I'm working in a small team of 5 developers at a startup. Our senior developer left about one month ago; he was by far the most knowledgeable of us in terms of the technologies we are using. His code reviews were always really good ones, as he didn't allow any code debt to get into production. During the months we worked together, I learned a lot from him and I'm really grateful for that.
Since he left, I'm really trying to help keeping the code quality as high as possible; basically like when he was part of our team. Thus, I'm doing code reviews in a thorough manner, trying to ensure the quality is on point. Here comes the problem: when this colleague puts code to review, I suggest him to try to do some changes for the better, but most of the times he responds like "I don't think this will help us", or "I don't think that is quite possible". This negativism and lack of confidence is getting on my nerves, especially when the solution I'm proposing is possible, but he spent only 5-10 minutes on investigating it to conclude it's not possible.
Another situation is when I suggest to add missing tests for a new piece of code, and his attitude is "Oh, not tests again .... ".
Similar remarks like these were also given by our ex senior developer, and I can feel that this colleague's attitude was a lot different than to my remarks. This attitude is not professional at all in my opinion, and I'm not sure what I can do about this. How could I proceed to make this situation better?