Has your son always preferred to speak in baby language or has that changed since your new baby was born? Does he speak in 2-word sentences with you and your husband and only speak in baby talk with others? Do you or your husband speak to him in a language other than English on a regular basis? Does he speak to his new baby brother/sister or a pet? If he speaks in baby talk to you, how do you know he's capable of 2-word sentences or does he go back and forth between them.
With language, as with everything else, children find their way at their own pace. And the presence of a second language doesn't interfere but it can appear that way to outsiders (i.e. anyone NOT the child!). PLUS, development is not linear. We go back and forth between things that we know and things we're exploring and things we feel safe with and things we'd like to try out. Do YOU and your husband talk to him a great deal throughout his day? DO you listen to him as you would any speaker and respond we you would with anyone? Do YOU speak baby talk to him? Or did you? Perhaps he misses that. Perhaps --he's only 2 after all--he thinks that baby talk is the way to talk. Do you speak to the new baby in baby talk and to him in people talk? He might not like that either. Or may not understand why you talk baby talk to one and no longer to him.
ANd DO READ READ READ to him AND his sibling every single day. YEs, even the baby. ANd what nice one-on-one time that makes for you and/or your husband and him. AND one-on-one with the baby as well. ANd that could easily evolve into your son "reading" to his baby. Adults tend to underestimate the importance of reading to infants and young children,thinking they don't get it. Just do it. Anytime. Read while they're playing cars. Read while he's sitting in the bathtub. And of course going to bed. And recite funny poems in the car. And play language games while you're waiting in line for whatever. Sandra Boynton's books are delightful for your children.. My boys can still recite with me (with far less pleasure than when they were little, of course.) Moo Baa LALALA....They're now 13 and 11. And let him see you reading, too. He can have his book and sit next to you while you each read. Of course, eventually (like in 2 minutes) he'll want you to read to him. Enjoy. I still read every night to my children.
For a 2 year, 3-month-old child to be able use 2-word sentences, it's unlikely that he's "delayed." And I do wish adults would relax a bit around children and let them emerge at their own pace. Certainly, if at 3, he's still preferring baby talk and hasn't progressed, you can consider evaluation, but he's 2. AND he has a new baby. ENjoy his language development as it occurs. Be aware of HIS patterns. And if you're still concerned in a year, an evaluation might be appropriate.
I taught French, Italian, ESL, linguistics, language development and methods classes for several years before I had children. I do not consider myself an expert. I learned more from being a mom and an attentive aunt than from the textbooks. Trust your instincts and your experience. Be honest with yourself in asking if you're giving him the attention he needs and deserves at such a tender age. They're lovely creatures, these 2-year-olds and they have so very much going on. Their language development is a miracle to behold and you can learn so much just from watching them grow. Keep a journal. That will help you keep track of his development and should you need it talk to a professional when the time comes.
Enjoy. Many blessings. Contact me if you'd like.