Just to ease some of the undesexed worries, the majority of dogs in the past 10 years or so have not have the tattoo's... in fact in the 14 years I had in the veterinary industry, only one vet clinic tattoo'd and only when the client requested it.

I would suggest that the small lumps you are feeling are his glands and usually when he is excited, these glands usually settle down around 12-18 months after desexing, but sometimes they are there for anxious dogs, like any excitement can cause them to be swollen. The should be on either side of the penile shaft near the middle of where the hind legs meet the body... if that makes sense?