At first glance of your question, I was going to recommend a list of man cave items, but I'm thinking differently now that I see you want to get a cool gag gift for your pastor. For starters, there are some really cool t-shirts that have funny over-the-hill type sayings on them, such as 'Made in 1975' or 'In dog years, I'm 280.'  Another idea, and this might sound random, but a fanny pack would be a good gag gift in my opinion, since he's an old pastor now at 40 and everything. Or, you could get him a box of Depends.
There are also some t-shirts that say 'Vintage 1975 - Aged To Perfection.'  Is he a grandpa yet? If not, anything that says grandpa on it could be given as a gag gift. I saw the term 'Fortylicious' on a coffee mug and thought you should actually make him a shirt. You can put 'Fortylicious' on it or something funny, you know, using puffy paint, and make him wear it while preaching the sermon after his birthday. Or, at least he can wear it during the bonfire party. That way, you could actually put anything on the t-shirt you want.