Call the bride and fill her in. Your situation has changed, so it's time to change the game plan.
It's probably not as big of a deal as you would think. If the groomsmen are wearing black tuxedos, then your son should wear black dress pants, white dress shirt and a tie that coordinates with the wedding colors. Because it's a Spring wedding, having a young child without a tuxedo jacket REALLY isn't a big deal.
If she really wants him in a tuxedo, then let her know that he will be in a black tuxedo that you will find at a reasonable cost. I just googled and here's one for under $50. You can get black dress shoes at Payless or Target for $20 and he will look great.
http://www.tuxedosonline.com/boys_tuxedo_black_infant_tod...
My son is in my BIL's wedding in September (as are the rest of us)... holding my breath to see what this is going to cost us too!