Well, I think the trends would suggest that people would start using this for a girl (think Addison, Emerson) but I personally like most last-names-as-first-names on boys.
Tennyson doesn't have bad connotations, but very literary ones (obviously) so if you think he would be a jock, it could be considered a bit weird. Of course, if he were artsy it could be a bit much on him as well.
Nicknames...are tough. I guess that's good if you don't want to use a nickname!
Tenny doesn't sound great, and reminds me of Penny, which is feminine. Tenn/Ten would be okay I suppose, but it's...a number.
It would help to know your other boys' names to see if it melds well with theirs. Same with your last name.
On the whole, I would pick something else unless you absolutely love it.
Others to try on: Sidney, Keats (if you like the poets!), Henry/Henric, Edison, Ephraim, Evren, Terrance/Terence, Tiernan