By the sounds of it, you showing pictures to Declan is a great move! It wont be a total suprise to see what his daddy looks like. Its understandable how worried you are but I think that if there is any unsureness (is that a word?), it wouldnt take more than a couple of days at most to get re-acquinted. Also a great idea about going away together for a special family bonding holiday.
Good luck, I hope all goes well!