The cost of the holding is £100,018 as you say and the shares are pooled in the same way as quoted sahres are. As you have 33 shares, the cost for CGT purposes for each one is £3,030.85.
As you are giving away shares to your son, the deemed disposal proceeds will be the value of 12 shares at the time of the gift. The gift will also be a potentially exempt transfer for Inheritance Tax purposes, the value of which will stay in your estate for seven years.
You may be able to make a claim for business asset holdover relief. The qualifying criteria are set out in HS295 here.
I hope this helps but let me know if you have any further questions.