Which one should I get him

[TwistedMister]

Are the full circle rings more secure then the half moon shape?

No. The solid rings require that the testicles be pushed thru, then the penis- if it can be pushed thru, it can be pulled out. A hinged ring, or a multi-part ring can be smaller, not allowing 'pullout'...assuming that the smaller size can be worn without too much trouble (not extremely painful, no abrasions/bruising, not too much sleep loss). It may take significant conditioning over time to be able to wear smaller sizes.