The musician wants the couple's 15-month-old son Bronx Mowgli to have a brother or sister and can't wait to start trying.
He said: "We are very lucky and hope to have another child at some point. We also like doing what you have to do to have a baby!"
Pete - whose band Fall Out Boy are believed to have split up - insists fatherhood is the best thing that ever happened to him and he has learnt a lot from raising Bronx.
Speaking at Hard Rock International's Rock and Soul Party in Miami, he added: "My child has taught me selflessness, patience and how to give this unexplained kind of love. I can't explain this kind of love, but Ashlee - who is just an awesome mom - shares this feeling with me."