Plain is best!! Make sure the thickness of the noseband makes the most of your cob's head. Mares, geldings and youngstock can be shown in a white halter, do you have any pics of your boy's head??
if you think he may be strong in a halter - mine sure is! - I would go for a plain flat noseband, it's workmanlike and will suit his head, good luck with him, he looks a nice lad!
I'd go for a white halter. You can get ones with chains on them if you think they are going to be strong. The Welsh D that i show is very strong in a halter but respects the chain enough that i dont have to worry too much about it too much, i do however have to worry about keeping up with her!