If he is struggling because he is refusing to try, and whining just because you are asking something of him, that's different.
This what he doing. He better than he was. He just knew before that this work at home and if he defiant at home, Mom is physically not able to put in time out or make get dressed. She developed pattern of negotiating and it used take hours to even get him in car for small reason like it not the shirt he wanted. He refuse try lot at home and so he behind with some skill because he never bothered trying.

He can recognize shapes, colours, and has good verbal skill but he just resistant to anything he not want do and stubborn in sense he thinks he can wait it out and win. Because eventually Mom needed to have him dressed and in car, he knew that refusing get dressed would eventually mean a reward or bribe for co-operating. He getting better since I spend lot time text supporting Mom when he have tantrum at home. She not able to place in room and was fearful he would hurt himself as he got more cross but Mom did manage let him get on with it and not react as he picked options that normally she would panic about.

He refuse get dressed one day and so she closed bathroom door and sat on floor to wait it out. He climb in bath and she frightened he going slip but she texting me and I told her let him. If he slip, natural consequence of climb in bathtub. He then start tipping out shampoo. Again, I tell her ignore him, he just acting in way to get response. Only word she said in that whole time was repeat instruction for him to get dressed and they not going anywhere until that done. He then try and make her guilty by say he hungry and want breakfast. She just tell him as soon as he dressed, they can do that. Eventually after 3 hour he realize that whatever he did, it not going to change requirement. It take lot of reassurance back and forth me telling her that she had to win this and not give in. He now learning he not rule the roost but he so behind in some thing because he refuse do in past.