K.,
as a mother of 4, a grandmother of 7, and a public high school english teacher for the past 18 years, i only have one question for you...who's the boss in your house? Tyler or you and dad? you may not like the responses he gives you with your new will and perserverence, but it only takes a few days with one so young to change the behavior. but be strong, you are the one in control. don't buy into his excuses and tantrums, stay firm, follow established rules and their consequences to the letter, and don't forget to reward
good behavior with words and alone time with tyler. no goodies and promises. let him help around the house with the new baby, picking up, doing small errand for you (i.e. go get me the baby's blanket, please.) be sure to play with him and READ to him. as a teacher, this is the single most important time you can spend with him. it will increase his vocabulary, speech, comprehension, and a joy of reading. read him anything, appropriate adult books you're reading, the newspaper, flyers, cereal boxes...it will help him IMMENSELY once he begins school.
i don't want to forget to mention about checking out his health. he sounds like a 2 1/2 year old, for sure, but make sure there is nothing medically wrong with him. if you have insurance, check out some behavioralists (ask your doctor for a list).
don't feel discouraged. you've just had a baby and you well know that it takes a year to get back to normal. find the small things in life to smile about, take lots of pictures and share them with tyler. and please, please, please, don't forget to include dad in all discipline, rewards, and alone special time with dad.
i hope this helps a little. keep your chin up, they DO grow up...sooner than you think. Good luck.