Can someone with previously undescended testicles father a child later?

Most likely yes. Despite damage to the undescended testicle and often a subclinical anomaly of the contralateral testicle, the majority of men will be fertile. Also, with modern advances in assisted reproduction, most men can father a child.
Mostly yes. Fertility potential after unilateral undescended testicle is very good even if it remains incompletely descended. Fertility rate for bilateral undescended testes is somewhat reduced but better the earlier they were brought down and lower to start with & less if were intraabdominal & later they were brought down. Fertility is totally absent for bilaterally undescended testes remaining undescended.
Depends. There are some risks of reduced fertility if abdominal testis are not located & surgically corrected in the first year or so. That said, there are some simple tests that his doctor or urologist can do on the sperm to see if healthy swimmers were present. I would consider him fertile for the time being until someone found evidence he wasn't.

Related Questions

My 5 month old baby is diagnosed with bilateral nonpalpable undescended testicles. What are the future problems associated. Can he have a baby?

Undescended testicle. The testicles must be found by ultrasound. If it is possible, they will bring at least one of the testicles into the sac and fertility will be maintained.