what can i take to make my bones stronger

I didn't know that.I did a cursory check using google.Apparently it is the vitamin D in cod liver,which increases bone strength.Slightly paradoxically cod liver also contains Vitamin A,which lowers bone density.
http://www.timesonline.co.uk/tol/news/uk/article563882.ece
I think you should check the brand of cod liver you taking.Many manufacturers nowadays reduce the level of Vitamin A in the cod liver.Also cod liver thins blood,which could be a problem in a high impact sport,such as boxing.