Ds1 is 5 and ds2 is now 15wks.
He wasn't so keen when I told him initially, he couldn't understand as we were already a family, so we told him that as we have so much fun with him we thought it would be nice to let someone else share the fun that ds1 gives to us, then we took a look at how babies are made (loads of books out there) then we went through his baby photos (many times) and we were honest, in that babies cry for communication, it's not all fun, but he could help his brother or sister learn as they got older.
We imagined what the baby might be like, and reminded him how much we loved him and that that it was OK to be nervous (ours was too) about the baby coming (I told him funny stories of times when I was good or not so good as a toddler too and that amused him,)
Do you have siblings so can relate to how you might have felt when younger, we did this too (both DH and I are the eldest.)
When the time came and ds2 arrived, ds1 was very quiet for a few days, couldn't give him eye contact then suddenly something clicked (we kept up the cuddles etc and reminding him how much we loved him) and within a few weeks he completely fell in love with his brother and still bosses me around on what he thinks his brother needs
I am bfeeding too (mixed feeding) and that was a source of curiousity but it didn't put him out especially as to start with he could help by holding his hands apart as I got ds2 latched!!
When are you due? I found as time went on in the pregnancy he did warm up more to the idea.
Has the teacher said anything about any changes?