Bridging loans, what to be careful of

If you have solid equity in your home and second home why cant you just guarantee your sons house purchase.

That aside you both need to be realistic about the sale price of your properties any interest will need to be paid on both properties until cash comes in. If the son's new property needs work, is it liveable in the meantime, are they prepared to move in?
Be very careful, building work can take longer and cost a lot more that most think, do not wear rose coloured spectacles when you look at your sons plans

My daughter and family wanted to move they also chose houses that needed work and wanted help to buy them, I rejected the first 2 they were wrecks needed a lot of work before they moved in. The third needed work but was liveable, they moved in, it took 10 yrs to complete but they made lot of money when they sold it

If you are getting the loan and then giving the money to your son for the purchase, you will have to go through the anti money laundering checks. These involve authentication of your identity, paperwork regarding the funds and their transfer.
You may need to consult a Financial/ legal advisor, if you haven’t already.