Can you please suggest me a good model-based learning algorithm to recommend items to the user? Is there any open source implementation available on model based learning algorithm? I am sure Apache Mahout doesn't implemented any model based learning algorithms.
If you set your recommender system up as a matrix factorization, then there will be plenty of Mahout experience out there, e.g. https://mahout.apache.org/users/recommender/matrix-factorization.html or http://de.slideshare.net/datasciencelondon/matrix-factorization-mahout-special-session
If you have a problem with expressing a recommender system like this, then have a look at this question & answer: https://datascience.stackexchange.com/questions/10114/using-movielens-dataset-build-recommendation-engine/10118#10118
Ask please if you have further questions. I won't be able to help you with Mahout but with the MF part.
There are several opensource offerings including Mahout, Lenskit, Crab, Duine,etc.
Here is an overview from ibm.com: