His solicitor has said that, if my stepfather were to die in the next 7 years, this would affect his IHT position and potentially result in a higher IHT bill.
This is possibly correct; depends upon the numbers.
Your step-father appears to have a "qualifying interest in possession"; this means he is entitled to the income of the trust. However, for IHT, it also means he is deemed to also own the trust property (which of course in reality he does not).
Thus, were he to for example surrender his interest he is deemed to have made a gift (ie a PET) of the trust property. If he survives 7 years no IHT and no problems.
If he dies within 7 years then the amount of the gift is taken into account in working out his nil rate band at the date of his death. Because of the gift his nil rate band (normally £325K) will be reduced thus increasing the IHT payable on his estate (ie the assets he owns at death). If he had not made the PET then he would have had a greater amount of nil rate band on his death and a lower IHT charge at that time.
Assume at his death his estate is 400K. IHT 40% x [400k - 325k] ie 30K.
However, if he died within 7 years of PET (of say 100K) then IHT would be:
40% x [400K - [325K - 100K]] ie 70K.
Also you are liable for any IHT which may arise on the PET itself.
This, as you say, is not in "real life" a gift from s-father but for IHT it is effectively treated as such.
What he seems to be saying is that you do not inherit until his death; however he's happy for you to inherit now but only if you indemnify him re extra IHT he may have to pay on death.
Theoretically, your exposure is up to a maximum of 40% of 325K ie 130k.