How to automatic disable static route in ipsec tunnel when tunnel goes down

why not just delete the static route, which will take precedence always?

other options would be to either do a automation stitch to delete/disable the static route when a event for the ipsec is down or https://community.fortinet.com/t5/FortiGate/Technical-Tip-Link-Monitor-Explained/ta-p/197504 

With FGTs, even if you put the same admin distance on the static route with BGP, like 20 for eBGP/200 for iBGP, it doesn't place both routes as parallel routes in the routing table. Instead, the newly introduced route wins.

You didn't explain the whole topology including both paths. But eventually the best option is to set up BGP through the primary IPsec path as well so that you can set local-preference to prefer primary IPsec path at the BGP level.

Otherwise you have to use link-monitor as @funkylicious suggests. It's quite ugly to have both static routing path and BGP routing path for the same destination when the static side is the primary. 

Toshi