I'm building a 7.5" 22 pistol for my soon to be 4 year old son and since there's no reason for the buffer tube or spring what can I put on the back of the receiver instead of the tube? Trying to make it as short as possible.

Been there, did that.  Ended up going SBR...good luck.

ETA:
That's a fairly large and heavy gun for a 4 year old.

My youngest is 11 now and he still prefers a Rem 597 Yellowjacket off the bench or a Glock with an AA .22 kit when he's standing and shooting.  The AR22 with a 7.5" barrel is still heavier than he likes and isn't as easy for him to shoot off the bench without short mags.  I did the Spikes plug 5-7 years ago and thought a 7.5" AR22 would be a cool gun for my kids when they were younger, but it was just a too large and heavy pistol for them to handle.  YMMV.

You can buy a receiver plug
Brownells  sells them Spikes other brands out there

4" barrel under the HG


sold out at the moment
http://www.avantlink.com/click.php?tt=cl&pw=55963&ctc=j0fzgz59vj00zk8a00y51&mi=10077&url=http%3A%2F%2Fwww.brownells.com%2Frifle-parts%2Freceiver-parts%2Fbuffer-tube-parts%2Fbuffer-tube-hardware%2Far-15-m16-22lr-pistol-plug-prod54594.aspx

Model 1 Sales also sold a buffer tube hole screw in filler part in the past.  May still for all I know.

They still have them. Lower reciever endcap.

Spike's pistol plug, with sling mount.

http://www.spikestactical.com/st22-pistol-plug-p-70.html

Thanks guys!

Just bought one of these on fleaBay... I thought it looked good enough for the job and cheaper than the alternatives.