Useful resources are the ‘Puberty’ series - Puberty Booklet. A good approach is to have boys learn about what also happens for girls (and vice versa). The Relationships, Sex and Other Stuff booklet is for teenagers (13 years or older).
The comprehensive publication Talk Soon. Talk Often. A Guide for Parents Talking to their Kids About Sex aims to increase the confidence of parents when talking to their children about relationship and sexuality topics.
Some other organisations that offer free or low cost resources include:
Sexual Health Quarters
SECCA (resources to support people with disabilities)
Freedom Centre (resources to support LGBTI young people)