What style look do you want for the boy?  If you really want a TUX just like your son then I'd suggest you go rent one nd save a big headache "cause you're looking at making a tucked shirt with the neckband & collar for a bowtie you also have to make, etc.  If you just want a SUIT for a 5-6 y/o or younger then look at Chery Williams Patterns, the Eton Ensemble--a jacket w/ no lapels and either long or short pants.  The boy's shirt would be with a small peter pan collar that lays over the jacket.  I've made this for several brides:  all ivory silk with pintucked shirt; brown linen suit with ivory tucked shirt and navy suit w/light blue shirt.  Eliminating the lapels saves a lot of work!  Whether a child's jacket,a man's or a woman's jacket, if you have lapels there is a lot of "inside work" on the jacket that is necessary to make it look good.  The little ringbearer will be cute in either style.  One thing that I have found consistently true on the main 4 brand of children's patterns is they are always too wide in the body.  I usually have to make the bodices by a smaller size to get the width of the necklines to fit better and then lengthen the bodices and skirts.  On little girls dresses I most often fully line the bodices avoiding facings for a nicer look and I do use interfacing in the button area.  For the little collars, I use a self interfacing when sewing from batitste or silk.