First off sorry to hear about your dad.
I'm in the construction industry, I'm a Qunaity Surveyor. I can tell you right now you have won the case already with the cival claim.
Two types of battles here:
The criminal side, the one the HSE will persue.
The Civil side, the one you will persue.
I wouldn't be suprised if the Site manager, and the director go to jail for this, combined with a massive fine for the company, from the HSE.
Your dad should not have been anywhere near the building site without an induction to the site. The fact they assumed he knew it all is meaningless, they have to ensure anyone on the site is properly inducted before carrying out any works.
The HSE will decide if they are going to prosecute. This has no reflection on your claim though so don't worry, if they for some deluded reason decide not to prosecute, you can still claim.
I'm pretty sure the HSE will find the company in the wrong which only strengthens your civil claim.
As others have said get a local solicitor and let them go through it all with you, and NEVER go for a solicitor who trys to deduct fees from your payout, payouts are exclusive of claims. IE they claim their costs on top of the payout you are awarded.
I know it's no substitute but I'm pretty sure your dad will recieve a massive pay out, although I fully understand this will not make up for the long term injuries he has suffered as a result.